A "Security Principal" is an entity, represented by an object in the
directory, that has the ability to access directory resources such as, data
on drives, printers, objects in the directory itself, etc. Security
Principals are either assigned rights or inherit them.
In Active Directory, there are three recognized Security Principals: Users,
Computers, and Groups. In other directories, you might find that
Organizational Units are Security Principals as well (the subject of intense
debate at times).
-ds