URL Rewrite is a method of cleaning up your URLs. For example: if you have
http://site.com/orders.aspx?order_number=1234, you could use URL Rewriting to make it appear as
http://site.com/orders/1234.
This control uses Regex to create rules which allows for a lot of flexibility. The rule for the above translation would look something like this...
<rule>
<url>/orders/(.*)</url>
<rewrite>/order_number.aspx?order_number=$1</rewrite>
</rule>
The rules are setup in your web.config.
To make this work with non-aspx page (.html, .php, .jpg, etc) you will need to make changes to IIS (described in the codeproject article).