WPF/E is one of Microsofts latest "Hot" things. It's used to create very rich, cross-platfrom, interactive web applications.
It's fairly easy to get this working. Here's some things you'll need
Windows XP / Vista with .NET 3.0
Visual Studio 2005 (Orcas 'JAN CTP' does not work with WPF)
I could go into how to setup a web site with good old aspx files, but the information is already out there.. You can find it in this good article.
Hopefully I can get some better posts up about WPF in the near future (I'm just green on this technology right now :) )