Scott Guthrie has made Part 2 of his blog on this important addition to ASP.Net available on his blog. This carries on from his initial look at teh ASP.net MVC (Model View Controller) in part 1 and explores deeper into the internals of the routing architecture.
So I don't repeat any of it, you can find out all about it at: http://weblogs.asp.net/scottgu/archive/2007/12/03/asp-net-mvc-framework-part-2-url-routing.aspx