The Windows Phone XAML App generally uses 2 threads
- UI thread
- Composition thread
UI thread is actually the main thread in the Windows Phone App and one of the main functionality of this thread is the handling of the user input, creating the XAML objects etc.
The composition thread was a new addition starting from Windows Phone 7.5 which takes care of the animations, graphics etc. and thus offloads some of the work down by the UI thread and thus makes the UI more responsive with user interaction.