File Formats and Extensions

When downloading content from the Web, you will undoubtedly encounter many different types of electronic file formats. The way to identify the file type is by looking at its extension, typically expressed as a dot followed by 2 to 4 letters (.xxx). Why should you care? You need to identify the file type to know whether it will work on your computer and whether you will need a particular type of software to decompress, play, or view it. It's helpful to know about the most common file formats you may encounter, so here's a bit about them.

Most files will be either text, graphic, photo, audio or video files. Some may be compressed, others not. The most common compressed files are those with extensions like .ZIP, .SIT and .TAR. These extensions represent popular compression formats for the PC, Macintosh, and UNIX respectively. They may be a single file or groups of files bundled into a single archive. An archive file can contain any type of file, and often contains software programs with related documentation. Occasionally you may encounter files with multiple extensions like .tar.gz, which usually means more than one type of software was used to compile and compress the file.

The most common graphics file formats on the Web are those with the extensions .jpg .gif and .png The .jpg is short for JPEG, which is a popular compression standard for photographs and other images. The .gif extension stands for Graphics Interchange Format, a standard developed by CompuServe in the late 1980s. Both these graphics formats are platform-independent, which means you can view them on a PC, Mac or UNIX computer provided you have a viewer for them. More recently, the .png (Portable Network Graphics ) format has become popular as an improvement over .gif files.

For video, popular extensions are .AVI and .RAM for the PC, .MPG (short for MPEG), which is platform-independent, and .MOV and .QT for QuickTime movies. QuickTime was initially developed by Apple just for the Macintosh OS, but now plays on Windows and UNIX, too.

Popular audio file formats include .MP3 for both Mac and PC and .WMA for the PC. Other file formats include .AIFF for Mac; .AU for Mac and UNIX; .WAV for the PC; and .RA for Real Audio, a proprietary system for delivering and playing streaming audio on the Web.

All of the file formats found on the Internet can be broken into one of two types: ASCII format and binary format. ASCII files are text files you can view with any word processor. Binary files contain non-ASCII characters. If you display a binary file on your screen, you will see a lot of strange symbols and characters.

The following is a guide to common Internet file formats.

.html/.htm
The language in which Web documents are authored.
File Type: ASCII
This file type requires a web browser, such as Firefox, Safari or Internet Explorer, for viewing.

.txt
A plain (ASCII) text file.
File Type: ASCII
These files can be viewed with a word processor like Microsoft Word or a simple text editor like Simple Text or BBEdit for the Mac. For the PC you can use Notepad, a program that comes with the Windows operating system.