There are 2 file types use the .XAP file extension:
1. XACT Project
Project created with the Microsoft Cross-Platform Audio Creation Tool (XACT); saved in a text format and contains the intermediate form of the information produced by XACT; includes complete descriptions of wave banks (.XWB files), sound banks (.XSB files), and global settings (.XGS files).
XAP files can be opened and viewed in a text editor, but they should not be edited manually. Instead, project files should only be modified with XACT.
Updated: January 4, 2010
2. Silverlight Application Package
An XAP file is an application package originally created for Microsoft Silverlight (a web browser plug-in enabling rich media content) but now also used for Windows Phone 7 mobile applications. It includes a .XAML file named AppManifest.xaml and may include .DLL files referenced by the software. XAP files are stored in .ZIP format.
XAP files were commonly used for distributing Windows Phone 7 applications.
Updated: April 20, 2011