XAP is the file which is used to deploy the app to the unlocked windows phone 8 device or submit the app for windows phone store.
In simple terms, XAP is the output of the Windows Phone 8 project in Visual Studio and is a compressed files which contains assemblies, images, configuration files etc.
The XAP file also contains 2 more important application manifest file
- AppManifest.xml
- WMAppManifest.xml
These 2 important configuration files are generally located within the project’s properties folder.