The .JPG (or .JPEG) file extension normally represents an image file stored in a format developed by the Joint Photographic Experts Group. JPG files usually represent photos, and these files are compressed, thus saving space on a hard drive or other media. An image such as a BMP file that may take up 250k of hard drive space, for example, may be represented as a JPG image possibly only requiring 35k of storage.

The SCR file extension represents a Windows screensaver, the animations that may appear on your computer whenever it has not been used for a few minutes. Screensaver animations can be virtually anything from starfields to flying kitchen appliances to photos of your favorite travel destinations.

TMP files are usually temporary files that applications such as word processors, games, disk utilities, etc. create and eventually remove. Most of the time .TMP files just contain indecipherable gibberish only usable by the application that created them. However, sometimes the curious can find readable text inside certain .TMP files if opened with Notepad or another text editor.

The PPT file extension normally represents a Microsoft PowerPoint presentation, or slide show. PPT files are most often created with Microsoft's PowerPoint software, though some other software, such as StarOffice, can also read and modify PPT files.

The TTF file format, created by Apple, represents a TrueType font.

The PSD file format, created by Adobe, contains graphics/photos created by image editing software such as Adobe Photoshop. The PSD file format is popular as it can be read by Macintosh and Windows computers. Normally PSD files are shared by desktop publishers and image editors as in-progress graphics, and are not used as much for graphics distributed to end-users.

The Windows Registry is the main repository of configuration information for Windows and most applications. For Windows users, the .REG file extension represents text files that can be imported to change settings in this repository. In some cases this bypasses the Control Panel and other means of configuring program and application settings, and in other cases this provides ways of changing information that cannot be changed (or changed easily) through graphical interfaces.

Files with the .AIF or .AIFF extension normally contain audio data (AIFF is short for Audio Interchange File Format). These types of files are common on Macintosh computers, especially with professional-quality audio recording and editing software.

CFG files normally refer to configuration files for applications or your operating system. If you have Microsoft Office installed on a Windows machine, these may be referred to as "Microsoft Office Outlook Configuration Files", though other applications use configuration files with the .cfg file extension.

Files with the CPL file extension are normally Windows Control Panel applets, found by clicking on "Start" - "Control Panel". Each .CPL file represents a tool to perform different types of configuration, including:

RAR is another file format most commonly used on DOS/Windows machines. RAR files take multiple files and pack them into one file, making it easier for people to distribute games, applications, screensavers, photo libraries, etc. Later, the RAR file recipient unpacks the file into its original contents. In this way it RAR files are much like ZIP files.

In general terms, a file format in the PC world signifies what type of information is containing in a particular file. These are usually designated by the one to four (usually three) characters (the extensions) after the period in a filename.

The PL file extension often refers to scripts of applications written in the Perl programming language. Perl scripts are used on many websites to handle interactive features such as search engines, polls and surveys, order purchasing, and more.

The BAK (or BK) file extension normally represents files that are backup copies of other file. There is no 'standard' file format for BAK files. BAK files can represent virtually anything.

The MID file extension normally represents that the file is a MIDI music file. MID files can only contain music, no voices. MID files are different than files such as WAV files in that they do not contain sampled, or digitized, audio information. Instead, MIDI songs only contain several tracks, each of which designates instruments to use and notes to play in a song, as well as miscellaneous musical effects. Thus, MIDI files will sound differently depending on which sound card is used to play a file.

A file ending in ".TXT" usually designates that the file is a plain-text file. This can contain virtually any type of text - a recipe, documentation for software, a book report draft, or whatever.

The ".ARC" file format is similar to the ZIP or RAR file formats in that they usually are archives containing various other files. Though not commonly used today, you may see older software or software packages for Unix machine compressed using this format.

Files with an extension of ".DOC" are usually documents created from word processors, most notably WordPerfect or more-frequently Microsoft Word. DOC files can contain text, images, photos, or any other type of information commonly found in word processing documents.

A file ending in ".EXE" usually means the file is an executable, or program file. This contains code that the computer follows in order to perform a particular task. A word processor, such as Microsoft Word, is an executable file; when started (double-clicked), the computer reads the file and executes the contained instructions to start up a word processing application. The same can be said for game executables, spreadsheet executables such as Microsoft Excel, music player executables such as Winamp, and so on.

The LOG file extension normally means that the file is a text file containing log information created by a particular program. There is no specific format to LOG files; they may contain the dates and times a particular program ran, what tasks it performed, how long those tasks took, etc. And, LOG files are not always text files but may be in a proprietary format depending on the software program that created them.

The FNT and FON file formats usually refer to older Windows raster and vector fonts. Unlike TrueType font files (.TTF extension) that can be resized to virtually any small or large size, the older FNT and FON files are limited to specific sizes. Also, they may not look exactly the same onscreen and on paper.

A file ending in ".1ST" usually is named "READ.1ST" or "README.1ST". These files are normally located in archives such as ZIP or RAR files, or they may be located in an application's main directory.
If the above are true, usually the file is a text file that the application developer(s) believe you should read first before either unpacking the archive or running the application. Such files may contain release notes with important software updates, bug lists, software incompatibilities with various hardware products, etc.

The ANI file extension often refers to an animated Windows cursor. Depending on how you wish to configure your Windows machine, you can have a spinning hourglass whenever your computer is waiting for a task to be performed, a tapping hand whenever you move your cursor over a link in a webpage, or even something more outrageous such as a flying spaceship over your normal mouse pointer.

Files with an extension of ".ASC" are usually ASCII text files, similar to the TXT file format. For Windows, while double-clicking an "ASC" file does not normally open Notepad, these files can normally be viewed with such a tool.

The PDF, or Portable Document Format, is a file format created by Adobe Systems. This format represents a way to transmit brochures, booklets, tax forms, restaurant menus, software documentation, or virtually any other type of word processing or desktop publishing document. This file format is popular on most operating systems, allowing people to transmit all types of content without having to use a myriad of different file viewers.

Especially on the Internet and when looking at electronic mail attachments, such as inside spam emails, you may come across files with multiple extensions, such as:
readme.txt.exe
In this case, note that the final extension is the actual extension of the file. The "txt" is NOT the file's extension; it is "exe". This means that although you might think the file is a text file, based on the "txt" extension, the file is actually an executable, based on the "exe" extension.
Thus, if you were to double-click this masquerading file, instead of getting a text document opened with Notepad, you would actually be running a problem. And, unfortunately, in this case, the program might be a dangerous Trojan horse (computer virus) that could do damage to your system or others'.
Be VERY WARY of any file containing more than one extension! Although not true 100% of the time, much of the time files with multiple extensions were written to fool people into executing Trojan horses.

The DAT file extension refers to a data file created by an application or the operating system. This type of file can contain virtually anything, such as: * Application help text, such as for a spreadsheet * High score files for a game * Software configuration options, such as the default font used in word processor documents DAT files are application-specific. Normally only the application that created a specific DAT file can read the DAT file, unless the application developer has released information to the public on how to read such file. DAT files may be unreadable, yet opening them in a text editor might display some legible information.

Files with the AU extension contain audio data. This file format was created by Sun Microsystems. In this way, WAV files are similar to the WAV files, but are found most often on Sun, Unix, and Linux-based systems. Since they are uncompressed audio files they take up much space on a hard drive; thus, you will most likely only see them used for small sound effects rather than entire songs.

The DIZ and NFO file extensions, most used in files named filename.diz and readme.nfo, represent text files most commonly used in the bulletin board days that preceded the Internet. These files were used to describe the contents of ZIP archives. Though not commonly used today, you can still sometimes see them when downloading files from certain websites.

The ".BAT" file format is a MS-DOS or MS-Windows batch file. In layman's terms, a BAT file contains a series of instructions for the computer to perform, usually for computer or file maintenance. Without going into too much detail, some common commands you may find inside BAT files include: COPY - Copies a file, sometimes from one directory to another CD - Changes focus to a new directory to perform file operations there DIR - Displays the list of files in the current directory MD - Creates a new directory MOVE - Moves a file, usually from one directory to another RENAME - Renames a file Just as TXT files, you can open BAT files in Notepad or another text editor. However, make sure you open your text editor first, and THEN open the .BAT file from within that application. This is because double-clicking BAT files usually run their contents as commands. Since batch files can perform many different tasks, they can either be very useful or malicious in nature. For this reason, be VERY CAREFUL when receiving batch files through e-mail, instant messenger, or other questionable sources.

Depending on how your Windows computer is setup, file extensions may be hidden. Instead of seeing files containing a period and 1-4 character extensions, all you see are the filenames without the extensions. Or, some files may have extensions, but not all of them. For safety reasons, and to learn more about file formats, you should configure Windows to show all file extensions. Although the instructions for showing file formats differ depending on the version of Windows you are running, they are generally the following: 1. Open any folder on your machine. 2. Choose "Tools" - "Folder Options". 3. Click the "View" tab. 4. Uncheck "Hide extensions for known types". 5. Click "OK" to close the dialog box.

The BMP file extension normally represents that the file is an image. BMP files are normally used on Windows machines though many other operating systems have programs to view such files. BMPs can represent virtually any type of image imaginable, from photographs of cars to holiday clip-art. BMP files are made up of pixels, tiny dots that represent color in an image. Some BMP files may only contain black-and-white images (1-bit), others images that have up to 16 colors (4-bit), other 256 colors (8-bit), some 65,536 colors (16-bit), and others 16 million (24-bit). The more colors that can be represented in a bitmap the larger the file. You can shrink a bitmap's size to some extent and still retain much of the image quality. However, if you increase a bitmap's size, no new detail is automatically added. Thus, the more you resample a bitmap upward, the blockier and more jagged it will look as the pixels turn into larger blocks of the same color. This results in 'pixilated' low-quality images. Virtually any image viewer/editor on Windows machines, and many on other operating systems, can view and edit bitmaps. BMP files are "uncompressed", meaning that computers perform no operations on these files to reduce their file size. Also, it means you can open and resave a bitmap as many times as you want and the file will remain looking the same (assuming you do not adjust the color depth), which is not the case for some other image file formats.

The .ZIP extension signifies the file is a ZIP archive. This is a way that people can transmit a game, screensaver, application, electronic book, or whatever as one larger file instead of multiple smaller files. ZIP files are used often in DOS/Windows environments. ZIP files must be created with a ZIP file creation tool. These tools take a group of files and mash them ("zip" them) together into one "archive" file, sometimes reducing the amount of space the ZIP file takes on the hard drive ("compressing"). After the ZIP file is transferred, the recipient takes the ZIP file and "unzips" it, causing the original files to appear on their hard drive. Another advantage to ZIP files is that, in certain cases, they can be password protected. This way (theoretically) only the recipient, knowing the password, can unzip the file. Different zip/unzip software packages use different compression mechanisms that may be easier/harder to break.

WAV (WAVE) files are audio files playable via multimedia playback software such as Windows Media Player and other software available for your operating system. These files may contain spoken words, sound effects, music, or whatever. WAV files normally take up a lot of room on hard drives and are being used less nowadays, especially for full songs. Other formats can store music taking up a reduced amount of space.

The MOV file extension normally represents a QuickTime multimedia file. QuickTime is a movie/video/multimedia format developed by Apple and is often used in websites, mostly for streaming audio or video. Apple distributes the QuickTime Player, allowing Mac and Windows users to view QuickTime multimedia either inside or outside of their web browser. Though the QuickTime player is free, Apple does sell a professional version supporting full-screen video and other features.