is not a stand-alone language, per-say. WPF is, as it implies by the name, a UI framework, that is used to facilitate the building of the User Experience Layer in an XML based construct that is used alongside another programming language, usually C# or Visual Basic, though any .NET language can be utilized in theory.
WPF exists as a subset of .NET Framework types that are for the most part located in the System.Windows namespace. If you have previously built applications with .NET Framework using managed technologies like ASP.NET and Windows Forms, the fundamental WPF programming experience should be familiar; you instantiate classes, set properties, call methods, and handle events, all using your favorite .NET Framework programming language, such as C# or Visual Basic.
Just wanted to set that straight...