PAS has a few basic concepts that you must understand in order to develop PAS related code.

There are a few basic concepts used in PAS:

Credentials are a set of information which can be used to authenticate a user. This can be a login name and password, an IP address, a session cookie or something else.
user name
The user name is the name used by the user to log into the system. To avoid confusion between "user id" and "user name" this tutorial will use the term login name instead.
user id
All users must be uniquely identified by their user id. A user's id can be different than the login name.
A principal is an identifier for any entity within the authentication system. This can be either a user or a group. This implies that it is not legal to have a user and a group with the same id!