Apache's mod_rewrite module lets you do some truly amazing things with URL rewriting. Here are a couple of examples of what you can do.
Make it so if someone types http://www.foo.com/12345 , Bugzilla spits back http://www.foo.com/show_bug.cgi?id=12345. Try setting up your VirtualHost section for Bugzilla with a rule like this:
<VirtualHost 126.96.36.199> RewriteEngine On RewriteRule ^/([0-9]+)$ http://foo.bar.com/show_bug.cgi?id=$1 [L,R] </VirtualHost>
There are many, many more things you can do with mod_rewrite. Please refer to the mod_rewrite documentation at http://www.apache.org.