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. Authentication is the process of identifying who a user is. Typically, a user proves to the system who they are by entering a valid user name and a valid password. The set of all usernames assigned to users of a system or a service. For example, the PAS ID is the namespace for users of Penn's modem pool.