AAA stands for Authentication, Authorization and Accounting.
+ Authentication: Specify who you are (usually via login username & password)
+ Authorization: Specify what actions you can do, what resource you can access
+ Accounting: Monitor what you do, how long you do it (can be used for billing and auditing)
An example of AAA is shown below:
+ Authentication: "I am a normal user. My username/password is user_tom/learnforever"
+ Authorization: "user_tom can access LearnCCNA server via HTTP and FTP"
+ Accounting: "user_tom accessed LearnCCNA server for 2 hours". This user only uses "show" commands.