AAA (Authentication, Authorization and Accounting) is a framework for controlling access to the network. Authentication, for example, is done by a central server, typically a RADIUS or TACACS server. This allows you to centralize authentication instead of storing usernames and passwords on all your network devices. Once a user is authenticated, authorization will define what the user is allowed to do or not. On a network device like a router, that might be running certain commands or not. Accounting keeps track of all usage. For example, a wireless user where we track how long he/she has accessed the Internet so that he/she can be billed.