What's the difference between a worker thread and an I/O thread?

Just to add on to chadmyers... Seems like I/O Threads was the old way ASP.NET serviced requests,

"Requests in IIS 5.0 are typically serviced over I/O threads, or threads performing asynchronous I/O because requests are dispatched to the worker process using asynchronous writes to a named pipe."

with IIS6.0 this has changed.

"Thus all requests are now serviced by worker threads drawn from the CLR thread pool and never on I/O threads."

Source: http://msdn.microsoft.com/hi-in/magazine/cc164128(en-us).aspx