Smashing Magazine has a great intro to rewriting urls:
“Many Web companies spend hours and hours agonizing over the best domain names for their clients. They try to find a domain name that is relevant and appropriate, sounds professional yet is distinctive, is easy to spell and remember and read over the phone, looks good on business cards and is available as a dot-com.
Or else they spend thousands of dollars to purchase the one they really want, which just happened to be registered by a forward-thinking and hard-to-find squatter in 1998.
They go through all that trouble with the domain name but neglect the rest of the URL, the element after the domain name. It, too, should be relevant, appropriate, professional, memorable, easy to spell and readable. And for the same reasons: to attract customers and improve in search ranking.
Fortunately, there is a technique called URL rewriting that can turn unsightly URLs into nice ones?—?with a lot less agony and expense than picking a good domain name. It enables you to fill out your URLs with friendly, readable keywords without affecting the underlying structure of your pages.”