Apache HTTP server has a rewrite engine called mod_rewrite.
mod_rewrite has been described as "the Swiss Army Knife of URL manipulation".
There are also rewrite engines for other web servers such as Microsoft's Internet Information Server (IIS).
A detailed description of Apache's mod_rewrite is available on the Apache.org website.
The primary benefits derved from using a rewrite engine are:
For Microsoft's IIS:
IISRewrite from Qwerksoft
ISAPI_Rewrite from isapirewrite.com