In Simple words, WPF stands for Windows Presentation Foundation.
In detailed view WPF is a new library to create the UI for smart client applications. Most noteable point is : application does not use Window handles. It is easy to resize the UI and supports sound video.
Definition of WPF is : Windows Presentation Foundation (WPF) is Microsoft's development tool for Web applications and rich client applications.With WPF, developers can use XAML, the Extensible Application Markup Language, to create custom controls, graphics, 3D images and animations that are not available in traditional HTML implementations.