To add to Crash's answer both stand for HyperText Markup Language also known
as HTML. When the web was much younger the file systems only supported a
maxiumon of 11 character file names. There were up to 8 on the left and 3 on
the right side of the dot. That meant that there was no room for the "l" at
the end. Macs supported longer extensions but other operating systems would
simply cut off anything after 3 characters.
So you could say htm is a holdover from the 'old' days. As Crash said it
doesn't really matter which one you except for which extension you web host
has first (or as the only) one to serve as the default file.
Other than that it is personal preference. (Even Dreameweaver on my Mac has
..htm as the default file extension.)
--
Cheryl D. Wise
MS FrontPage MVP
http://mvp.wiserways.com
http://starttoweb.com
Online instructor led web design training - Next Session June 26th