An RT user is identified by their e-mail address and a username. Each user is also assigned an id by RT, but this id is typically hidden in the RT interface and is not usually used directly.
A user with the Let this user access RT is an active user, meaning they can login (if they have a password) and perform actions in RT. Uncheck this option to disable the user account.
Selecting the checkbox 'Let this user be granted rights (Privileged)' on the user administration page makes a user Privileged. It is also possible to set various configuration options to make new users Privileged when they are created. Privileged users can access the full web interface and can see and modify objects in RT based on their Rights.
A user can be a member of a Group.
A user with the global user right SuperUser can do anything.