SpaceGirl said:
.com files are executables
.exe files are too
http://something.com/ (or some other extension) = a web site.
http://www.domain.com/index.com would be a REALLY bad name for a page.
http://www.domain.com/index.htm would be a lot better
http://www.domain.com/index.html would be even better as far as I'm
concerned, since ".html" is the proper abbreviation of HTML (the "L"
is part of the name of the format). ".htm" is a bastard Microsoftism.
However, if index.htm[l] is the default index of the directory (as it
usually is), it would be even better to reference it as
http://www.domain.com/, or, if using a relative reference, as "./" (a
dot followed by a slash), thus creating a reference to the directory
index without the filename hardcoded, as I discuss here:
http://webtips.dan.info/subdir.html
On the other hand, file extensions have no official meaning in Web
standards, so you can theoretically serve an HTML document from a URL
containing a ".com" file extension, and it will be standards-compliant
if sent with the proper MIME type. However, certain inferior (but
popular) browsers violate the standards and second-guess MIME types
based on things including the file extension, and hence might mess up
if you do things like that.