WPF vs Silverlight

That's an extremely broad question. My company recently wrote a whitepaper outlining the differences between the two technologies, and it's around 70 pages. Unfortunately, it's not published yet, or I'd give you the link.

EDIT: As promised, here's the link to the whitepaper on Codeplex:

http://wpfslguidance.codeplex.com/

However, I'll try to summarize.

1. WPF is a thick Windows client platform that has access to the full .Net Framework. Silverlight is a browser-based technology that has access to a subset of the .Net Framework (called the CoreCLR). So, you'll notice differences using seemingly every day methods and objects within the framework. For instance, the Split() method on the String class has 3 overrides in Silverlight, but 6 in the .Net Framework. You'll see differences like this a lot.

2. Within WPF, all visually rendering elements derive from the Visual base class. Within Silverlight, they do not; instead, they derive from Control. Both technologies, however, eventual derive from the DependencyObject class up the hierarchy.

3. WPF, currently, ships or has available more user controls than Silverlight; though this difference is being mitigated through the Silverlight Toolkit and the upcoming release of Silverlight 3.

4. WPF supports 3 types of routed events (direct, bubbling, and tunneling). Silverlight supports direct and bubbling only.

5. There's quite a few data-binding differences that will be somewhat mitigated with the next version of Silverlight. Currently, Silverlight doesn't support the binding mode, OneWayToSource, or Explict UpdateSourceTriggers. In addition, Silverlight defaults to OneWay databinding if none is set, while WPF uses the default mode specified by the dependency property.

6. Silveright doesn't support MultiBind