Explanation
http://www.cisco.com/en/US/products/ps6638/products_data_sheet09186a00804fe332.html
Need for AAA Services
Security for user access to the network and the ability to dynamically define a user's profile to gain access to
network resources has a legacy dating back to asynchronous dial access. AAA network security services
provide the primary framework through which a network administrator can set up access control on network
points of entry or network access servers, which is usually the function of a router or access server.
Authentication identifies a user; authorization determines what that user can do; and accounting monitors the
network usage time for billing purposes.
AAA information is typically stored in an external database or remote server such as RADIUS or TACACS+.
The information can also be stored locally on the access server or router. Remote security servers, such as
RADIUS and TACACS+, assign users specific privileges by associating attribute-value (AV) pairs, which
define the access rights with the appropriate user. All authorization methods must be defined through AAA.