Typically, a user proves to the system who they are by entering a valid user name and a valid password.
Authorization is the process of identifying what a user can do.
For example, after logging in to a system, a user may try to issue commands. The authorization function determines whether the user is permitted to issue those commands. In some systems, authorization and authentication are merged into a single process. Accounting is the process of measuring the resources a user has consumed. Typically, accounting measures the amount of system time a user has used, or the amount of data a user has sent and received.