They can represent a different type of server technology being used.
..htm and .html both mean standard html pages.
..php is a PHP script page
..cfm is a ColdFusion page
..asp is an Active Server Page
..aspx is an ASP.Net page
..jsp is a Java Server Page
..shtm and .shtml are pages that make use of server side includes.
..pl is a perl script
..cgi is an executable form processor that makes use of the common gateway
interface
The list could continue on still, but mostly they give an indication as to
the technology used, and let installed server-side applications perform
processing on them in order to return some sort of dynamic content.
Hope this helps,
Mark Fitzpatrick
Microsoft MVP - Expression