WPF is the abbreviation for Windows Presentation Foundation. It is a UI framework to build desktop application with taking advantage of graphics hardware. It provides resolution independent and vector based rendering engine to make UI faster.
WPF supported by .NET Framework 4 and latest .NET version(open-source). Latest version of .NET is a cross-platform but WPF only runs on Windows. As it is a subset of .NET located in the System.Windows namespace.
UI in WPF is generally specified in XAML(Extensible Application Markup Language). XAML is declarative language which simplifies creating UI for windows application. With XAML you are able to write and edit UI very easily.