WPF uses XAML. Silverlight uses XAML. WPF is mostly for writing client (Windows) apps, but XBAPs run in a browser. Silverlight is for the web, but an offline story is planned. It's easy to get confused. Worse, people often feel there is a "vs" aspect to it - Silverlight vs WPF. Mike Taulty puts it like this:
Of course, it's not a fist fight. Mike goes on to elaborate on the continuum of technologies and the way you can share skills between them. He also points to a post by Karl Shifflet that demonstrates how code can be reused between WPF and Silverlight applications.
A vital reference for anyone who is moving around on this continuum is a whitepaper (with companion code) that highlights the similarities and differences between WPF and Silverlight. It's available on Codeplex now. It's full of detailed summaries like this:
Check it out!