Windows Presentation Foundation ("Avalon") applications have two modes of execution: standalone applications and in-browser applications. The former model is I think well-understood, but the browser-based capabilities of Avalon are new to some people. Simply put, Avalon web browser applications (we're still trying to come up with a good name - any suggestions?) run within the context of a partial trust sandbox, either filling the whole browser window or an inline frame. They're perfect for enterprise applications where you don't want to deploy anything to the client (simply browse to http://myserver/myapp.wba) and they'll also be great for really immersive web experiences as Avalon becomes more ubiquitous on the desktop.
Karen Corby has just written a good whitepaper on this topic; it's currently going through the MSDN edit / production process, but in the meantime I wanted to give you all an early peek of the article. You can download it from the staging server here.