.html - Hypertext Markup Language: Basic web pages are coded in html, or more recently, xhtml (Extensible Hypertext Markup Language)
.htm - Adapted for the Windows/DOS Operating System which required 3-character extensions
index.html / index.htm / default.asp / default.aspx - First page of a website
.xml - Extensible Markup Language: Used for data-interchange such as rss feeds
.gif - CompuServer Image: Use for logos and other graphics that contain minimal colors
.jpg - JPEG: Use for photographs and other images that contain many colors
.png - Portable Network Graphic: Newer web graphic format. Better compression, color, and transparency, however, older browsers will not read .png files
.swf - Small Web Format: The (published) web file used for flash animation / flash movies
.asp - Active Server Pages: Default.asp is used instead of index.html for the first page of the website. Runs on various Windows operating systems and connects to Microsoft Access, SQL, and MySQL databases.
.aspx - ASP.NET: Aspx is the successor to asp and runs on the newer Windows operating systems
.cfml / .cfm - ColdFusion Markup Language: Built using the ColdFusion server-side content management tool and connects to Microsoft Access, SQL, and MySQL databases
.cgi - Common Gateway Interface: Used to execute server side programs and can be written in Perl, C, Python, etc. and are typically found in the /cgi-bin directory
.pl - Practical Extraction and Report Language / Perl: A CGI programming language designed for processing text
.php / php3 - PHP: Web scripting program which runs on either Linux or Windows as long as the PHP compiler is installed
.shtml / .shtm - Server Side Includes / SSI: HTML pages built using include files which run on web server must have SSI turned on
As the web evolves, new products become available, so tomorrow you may find a new extension!