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. PennNet Authentication System - used for authenticating and authorizing modem pool users, as well as authenticating Penn users for another of other campus services.