If you have done much work with Apache then most likely you have experienced a need for enabling the rewrite engine. If you are unfamiliar with the rewrite engine, you can think of it as a means for your web server to literally rewrite urls so they are easier for users to remember More here

Of course the rewrite engine serves as much more than just a means for your users to more easily remember addresses. Many server softwares (such as the social networking server Elgg) depending upon the rewrite engine in order to function