Your Webmaster decides what the default file is, but it's usually index.htm OR
index.html. For security/utility/convenience reasons though,
many sites are configured to look for one
specific file name as the "primary" file in a Web directory. This is the file
that the server displays first when someone links to your URL. If it requires
a specific file name, this is usually either index.htm OR index.html (but not
both).
If an html document contains "Server Side Includes" that means that it includes
special commands for the Server to process. This is normally how things like
counters are done on basic web pages.
On some websites, the webmaster sets up the server to parse, or read
through every .html or .htm file to see if it contains server instructions.
However, this WILL slow things down when people link to the pages.
Webmasters who are concerned about server performance commonly use the
.shtml extension for an .html file that must be parsed.
clipped from: www.hwg.org