With the recent release of URL-rewrite module for IIS 7.0 and the inclusion of ASP.NET routing into the .NET Framework 3.5 SP1, many ASP.NET developers are questioning how these two features relate to each other and when each should be used.

Ruslan Yakushev has an instructive post on LearnIIS.net.

The essential difference is that IIS URL rewriting is handled at a lower level than ASP.NET routing and is invisible to the client.

Ruslan provides a visual workflow of the IIS 7 URL Rewriting process. You'll see that the URL Rewrite Module is activated before the request is passed to a request handler, e.g., the ASP.NET managed ASPX handler. IIS URL rewriting is unaware of specific request handlers.

He also provides a visual workflow the ASP.NET Routing process. You'll see the ASP.NET routing is a request dispatcher and must be fully aware of which handler a specific request should be routed to.

From Ruslan's description: