<p>Looking at the processmodel element in the Web.Config there are two attributes.</p>

<pre><code>maxWorkerThreads="25" 
maxIoThreads="25"
</code></pre>

<p>What is the difference between worker threads and I/O threads?</p>

Fundamentally not a lot, it's all about how ASP.NET and IIS allocate I/O wait objects and manage the contention and latency of communicating over the network and transferring data.
I/O threads are set aside as such because they will be doing I/O (as the name implies) and may have to wait for "long" periods of time (hundreds of milliseconds).  They also can be optimized and used differently to take advantage of I/O completion port functionality in the Windows kernel.  A single I/O thread may be managing multiple completion ports to maintain throughput.
Windows has a lot of capabilities for dealing with I/O blocking whereas ASP.NET/.NET has a plain concept of "Thread".  ASP.NET can optimize for I/O by using more of the unmanaged threading capabilities in the OS. You wouldn't want to do this all the time for every thread as you lose a lot of capabilities that .NET gives you which is why there is a distinction between how the threads are intended to be used.
Worker threads are threads upon which regular "work" or just plain code/processing happens.  Worker threads are unlikely to block a lot or wait on anything and will be short running and therefore require more aggressive scheduling to maximize processing power and throughput.
[Edit]: I also found this link which is particularly relevant to this question:
http://blogs.msdn.com/ericeil/archive/2008/06/20/windows-i-o-threads-vs-managed-i-o-threads.aspx