These are file extensions or formats you're likely to come across at one point or another if you're going to get into designing web sites. They're split into common groups, include a brief description of where you're likely to see or use them, and clicking the *.EXTENSION link in the titles will take you to a Wikipedia entry with a lot more details on that file format.
This list is a collection of about 45 file formats I've used enough times to remember when presented with a huge list of file formats after roughly five years of experience.
Browser File Formats
Browser file formats are files that instruct web browsers to do something directly. Many technologies may work with browser file formats, but browsers are generally the things that make use of them.
*.AS Adobe/Macromedia Action script file
AS files are generally used as library or optional include files in Flash or Flex applications.
*.CSS Cascading Style Sheet file
CSS files contain CSS rules which web browsers use to decide what web pages should look like.
*.HTM / *.HTML Hyper Text Markup Language
HTML files are web pages. HTML is what files such as PHP, ASP, CFML, and JSP produce when they're executed.
*.ICO Icon file
ICO files are commonly used as Favicons (that little icon in the address bar)
*.SWF Adobe/Macromedia Shockwave Flash file
SWF files are generally used in Rich Internet Applications and require the Adobe/Macromedia Flash Player to view.
*.VBS Microsoft Visual Basic Script file
VBS files are similar in concept to JS files but they often have control over operation system functionality normally off limits to JS, however VBS files are only supported by Microsoft Internet Explorer versus JS which is available on almost every popular web browser. VBS is generally only found on private Microsoft Windows based Intranet sites, though even there it's being replaced by newer offerings from Microsoft.
Image File Formats
Image file formats are files used to store and transfer photographs, vector art, logos, favicons and things like that. They're also used to store information artists use to modify the images in image manipulation applications such as Photoshop and Fireworks.
*.BMP Bitmap Graphic file
BMP files are an early image format with little or no compression applied. BMP files are sometimes found on websites, but have generally been replaced by formats such as PNG, GIF, and JPG in recent years.
*.GIF Graphic Interchange Format file
GIF files are image files that sometimes include simple frame-based animation and alpha transparency. PNG has gained popularity over GIF in recent years do the the superior color quality of PNG, however transparency problems with Microsofts Internet Explorer 6 have slowed the transition from GIF to PNG.
*.JPG / *.JPEG Joint Photographic Group image file
JPG files are image files designed specifically for photos. The compression method used has been optimized for working with complicated images, making it especially well suited for transferring photos on the web.
*.PNG Portable Network Graphic file
PNG is an image file format that has gained popularity over GIF in recent years do the the superior color quality of PNG, however transparency problems with Microsofts Internet Explorer 6 have slowed the transition from GIF to PNG. PNG is also the native file format used by Adobe/Macromedia Fireworks, similar to how PSD is the native file format used by Adobe Photoshop. PNG has the ability to store information about layers when used in Fireworks in much the same way that PSD can store information about layers in Photoshop.
*.PSD Photoshop Document file
PSD is an image file format and is the native format used by Adobe Photoshop. PSD allows image authors to store information about layers in an image when edited in Photoshop in much the same way that PNG enables layer information when edited in Fireworks. Unlike PNG which generally has multiple export modes and can be exported for use on the web, PSD is primarily used for raw or drafts before exporting the finished pieces of the image to other image formats such as PNG, GIF, or JPG.
*.SVG Scalable Vector Graphics file
SVG files are vector image files. Unlike common raster image formats that store information about each pixel in the image, vector image formats only store information about how to draw the image, which generally consists of coordinates to place simple shapes and how to fill them with color.
Audio/Video File Formats
Audio/Video file formats are used to store and transfer audio and video.
*.AVI Audio Video Interleave file
AVI files are commonly used to distribute video.
*.FLV Flash Video file
FLV files are usually not observed directly, but rather played through an SWF file. FLV files are used for distributing streaming video.
*.MOV / *.QT Apple QuickTime Video Clip file
MOV or QT files are used to distribute video, similar to AVI files. Quicktime is generally known for a great level of quality in video.
*.MP3 MPEG Audio Stream, Layer III file
MP3 files are used to distribute audio. MP3 files use a compressed version of the audio information and are generally many times smaller in size compared to WAV files, despite including extra information about the audio such as the author, title, album name, and description.
*.MPEG / *.MPG MPEG 1 System Stream file
MPEG or MPG files are used to distribute video, similar to AVI, MOV, and QT files.
*.WAV Waveform Audio file
WAV files are an early audio format file which has all but been replaced by MP3 on the web due to superior compression and information embedding provided by the MP3 audio file format.
*.WMA Microsoft Windows Media Audio file
WMA is an audio file similar to MP3 and WAV developed by Microsoft. Like WAV, WMA has been replaced to a large extend on the web by MP3.
Compression File Formats
Compression file formats are used to apply compression to single, or groups of files in order to reduce the size needed to store and transfer the files, as well as making large groups of files easier to transfer at once.
*.BZ2 BZIP2 compressed archive file
BZ2 files are generally used by programmers to distribute compressed source code packages.
*.GZ GZIP compressed archive
GZ files are sometimes used in the same way BZ2 files are, for programmers to distribute source code packages. Some systems use GZIP compression on their old log files to save space which results in file names such as "date.log.gz". Some web servers use GZIP compression to compress data before sending it from a server to a browser.
*.RAR winRAR compressed archive
RAR files are often used by individuals to compress and transfer large files, or large groups of files. Similar to GZ, BZ2, and ZIP files.
*.ZIP Compressed archive file
ZIP files are similar to RAR, BZ2, and GZ files however ZIP files are generally a lot more common. ZIP files package and to a point compress other files to be transferred from system to system.
Misc File Formats
Misc file formats are generally not designed specifically for web sites, but have found numerous uses on them.
*.*.BAK Backup copy of file
Backup. The second asterisk is generally the original file extension.Rather than trying to open this file, you will usually rename it and remove the .BAK from the end, then open it with the appropriate application
*.DOC Microsoft Word document
DOC files are often used by companies to distribute bundled information suitable for printing. DOC files are a favorite of companies for sending contracts via e-mail. DOC files are similar in concept to PDF files.
*.INI Information/configuration file
INI files are commonly used to store persistent configuration data for web servers and web browsers. INI files are also used in the same way for applications that have nothing to do with web design. Using XML files to store configuration data instead of INI files has been gaining popularity in recent years.
*.KML Google Earth/Keyhole place mark file
KML files are used to share specific locations, or placeholders in the popular Google Earth application. KML files are becoming increasingly popular on business web sites who want to provide directions for customers.
*.PDF Acrobat Portable Document Format file
PDF files are often used by companies to distribute bundled information suitable for printing, similar to DOC files. Some print shops will ask for PDF files when you need to have fliers or brochures printed out. Some companies will make PDF copies of important information found on their web site available for people do download and read offline, or print out and read away from their desk.
*.RDF Resource Description Framework file
RDF files are often used to deliver RSS content. RDF RSS feeds are generally more specialized in comparison to the generic XML versions of RSS files.
*.TXT Plain text file
TXT files are plain text files. TXT files generally display the characters as-is with no consideration for special style codes.
*.XML Extensible Markup Language file
XML files are generally used to store structured information. Many web sites use AJAX to load XML in the background and update small portions of web pages without needing the server to generate the entire web page over again with slightly changed information. XML files are also gaining popularity as a settings or preference file format. The generic yet descriptive format used to section data off makes XML the perfect file format for storing raw information that will be displayed in many different ways.
Server File Formats
Server file formats are generally scripts that web servers execute in order to produce web pages dynamically. these types of files are also often used to perform maintenance on the server through automation.
*.ASP Microsoft Active Server Pages
ASP files are an earlier version of ASPX files. ASP files are similar in concept to CFML, PHP, ASPX, RB, and JSP files.
*.ASPX Microsoft ASP.NET source code file
ASPX files are a more recent version of ASP files. ASPX files are used on web servers to prepare HTML documents to send to a web browser. ASPX files are similar in concept to ASP, CFML, PHP, RB, and JSP files.
*.CFM / *.CFML Coldfusion Markup Language file
CFML files are used on web servers to prepare HTML documents to send to a web browser. CFML files are similar in concept to ASPX, PHP, ASP, RB, and JSP files.
*.CGI Common Gateway Interface file
CGI files are generally used to provide a bridge, or gateway, between a web server and applications installed on the server such as Python, Perl, sed, and many others.
*.JSP Java Server Pages
JSP files are used on web servers to prepare HTML documents to send to a web browser. JSP files are similar in concept to ASPX, PHP, ASP, RB, and CFML files.
*.LOG Log file
LOG files are generally exactly what the name implies, a log. Web servers use them to store details about each visit to a web site as well as to store information useful to administrators and developers in the event of a problem. Old, or archived LOG files are often compressed using GZIP compression, which results in "filename.log.gz" types of file names.
*.MDB Microsoft Access Database file
MDB files are a Microsoft database data storage format. The MySQL database engine has been gaining popularity over MDB in recent years. There are many MDB to MySQL converters and conversion methods available on-line.
I posted a how to for Importing MDB to MySQL on a Debian server not too long ago.
*.PHP Hypertext PreProcessor script file
PHP files are used on web servers to prepare HTML documents to send to a web browser. PHP files are similar in concept to ASPX, JSP, RB, ASP, and CFML files. PHP is one of the most popular pre-processors used on the web today.
*.PL Perl script file
PL files are Perl script files. Perl is one of the first scripting languages to be used on the web. Perl is often used in conjunction with CGI. In recent years PHP has stolen much of Perls thunder, leaving Perl used primarily by veterans.
*.RB Ruby script / Ruby on Rails class file
RB files are used on web servers to prepare HTML documents to send to a web browser. RB files are similar in concept to ASPX, JSP, PHP, and CFML files.
*.SHTML / *.SSI HTML including Server Side Includes
SHTML files are used on web servers to tell the web server that there's lines in the file that need to be replaced with the contents of other files before sending the contents of the file to the web browser. SHTML is generally a very simple version of what file types such as ASP, ASPX, CFML, JSP, PHP, and RB are designed for.
*.SQL Structured Query Language data file
SQL files generally contain the description of a database and the database table layouts. In some cases SQL files will contain information dumps from database tables, though this is generally reserved for small databases.
.htaccess Apache dynamic configuration file
.htaccess files are commonly used to make runtime adjustments to an Apache web server configuration without needing to restart the Apache server. Not all configuration options are able to be altered via .htaccess and depending on the servers configuration at start up there might not be any options that can be configured via .htaccess