A basic xap file in silverlight will have an assembly related to specific code for the application, an application manifest file and any additional assemblies need to run the application. At a minimum, two files are needed, the application manifest file and the application assembly. For example:
Once you have created the .xap file, the Silverlight 2 or 3 plug-in downloads the file and runs it in a separate work space.
A .xap file is used to contain and transfer the assemblies and resources of a managed code application. This managed code application must be run within the Silverlight 2 browser plug-in.