Over at The Jakol's Den there's cheat sheet for .htaccess that
shows you some of the more common usages. For those of you who
don't know, .htaccess is a small configuation file that Web
publishers running Apache can use to do all sort of neat things
on their sites. The short definition from wikipedia:  .htaccess
(Hypertext Access) is the default name of Apache's directory-
level configuration file. It provides the ability to customize
configuration directives defined in the main configuration file.
The configuration directives need to be in .htaccess context and
the user needs appropriate permissions.  It's actually much less
complicated than it sounds. There is much more detail in the
full Wikipedia entry as well as some common usage tips. There is
also a whole lot you can do with it, so references like this are
very helpful.