The user authentication is used to identify the user which is going to login in.The user authentication is usually done by setting cookies and then the user is tracked with the help of cookies so that he has not to enter password to view each page.
Whereas user authorization refers to the permissions provided to the user to view the particular page. On most of the smaller websites, these two processes goes hand in hand but in large website especially of organisations where a lot of user has to give specail permissions so that he can work with those powers has to be provided.
The user authorization can be an ip based where particular subnet is just to login as in the case of a particular department.
Authorizatoin can be on the basis of database where special table is prepared users and their permission levels.