Silverlight is a browser-based technology for developing rich internet applications, while WPF is for building desktop applications. Some key differences are:
1. Silverlight applications can be deployed across browsers and platforms, while WPF only runs on Windows desktops and in Internet Explorer.
2. Silverlight uses a lightweight version of .NET's Base Class Library, while WPF uses the full BCL.
3. Silverlight does not support some elements and features that WPF does, like tooltips, labels, and synchronous network calls.