Authentication is all about checking and validating that a user is who he says he is. Authentication is commonly based upon a username and a password (but from now on and in the future we should all really be heading towards certificates).
To set a daily task to the word “Authentication”, this is really what you do once you logon to the domain at work once you’ve booted up your computer.
Authorization determines wheter a user has access to resources – i.e. on a specific share on the company fileserver.
It involves SAM, ACLs (Access Control Lists) and Kerberos.