A principal represents the identity and role of a user and acts on the user's behalf. Role-based security in the .NET Framework supports three kinds of principals:
Generic principals represent users and roles that exist independent of Windows NT and Windows 2000 users and roles.
Windows principals represent Windows users and their roles (or their Windows NT and Windows 2000 groups). A Windows principal can impersonate another user, which means the principal can access a resource on a user's behalf while presenting the identity that belongs to that user.
Custom principals can be defined by an application in any way that is needed for that particular application. They can extend the basic notion of the principal's identity and roles.