There are two users plugins:
1) The low level one, which includes creating users in the users table, handling the user login and password, and generating a cookie with the user session information;
2) The high level one, which is the interface allowing interaction with users.
Both of these are core plugins so they will always be available.
The User Interface includes all the forms necessary to handle the user account creation, management, and deletion. It also comes with the user login and forgot password forms. The new account verification code. And a few other things too.