.htaccess

.htaccess files are used to customize a web site on an Apache server. Although .htaccess is only a file, it can change settings on the servers and allow you to do many different things, the most popular being able to have your own custom 404 error pages. .htaccess isn't difficult to use and is really just made up of a few simple instructions in a text file. This tutorial explains .htaccess and gives specific examples on how to write .htaccess files. Here is another tutorial.