What is a Beta in BF3 and Mw3?

Well as we all know a "war" is going on of which game is going to be better: MW3 or BF3. Yesterday I was looking through my friend's wall on facebook (He is a BF3 maniac) and this one poll said "which game should get the right to own a beta?". I wanted to vote, but I don't know what a Beta is. What is a Beta? (My vote is on BF3 being the best game!)