Personally this was one I asked myself a few days ago when I learned about the game. With the closed beta date of December 3rd coming up quickly how do I get a beta key? Well at first I was pointed in the direction of gamespot's site where all I had to do was log in to my account then click for a key. Soon after I learned that it was just a community key, so heres a brief explanation of the two.
They typically have (community name)_Beta_Key when you register them on your account. These keys give you a CHANCE to get accepted into 1 of the 3 beta phases. You could get very lucky and be accepted to all 3, but the odds are stacked against you. Also just because you get accepted to 1 of the betas doesn't mean you are included in the other two as well. The drawings are completely random. Unless you have a VIP key.
I don't have one personally and am hoping to get one. These give you guaranteed access to all of the closed betas and content testing the game will have before release. This key looks something like the community key, however it will have _VIP in the name as well. These keys are given away daily on Twitter and Facebook by the game's staff as well as on various sites such as this one. They are in high demand however and you must be quick if you want to get one.
If you have already put a community key onto your account and get a hold of a VIP one you don't need to make a new account. Just register the code on the same account and it will override the community key.