What is the difference between authentication and authorization ?

An authentication system is how you identify yourself to 
the computer. The goal behind an authentication system is 
to verify that the user is actually who they say they are. 
There are many ways of authenticating a user.Like
Password based authentication 
Device based authentication 
Biometric Authentication 

Authorization 
Once the system knows who the user is through 
authentication, authorization is how the system decides 
what the user can do.
A good example of this is using group permissions or the 
difference between a normal user and the superuser on a 
unix system.