First time I heard about XAML (pronounced "Zammel") was when I first heard about Windows Presentation Foundation (WPF). Then I also heard about it when Windows Workflow Foundation (WF) entered the scene at the PDC 05. I never understood how XAML was related to WF, since I thought it had to do with the presentation (WPF) somehow.

Recently I've learned that XAML is not bound to WPF in any way (even though it was originally designed for WPF), but rather a general-purpose declarative language for constructing and initializing .NET objects. It is possible to use WPF without XAML and XAML is also used without WPF (for example in WF). You can read more about XAML here.