well, there's open beta and closed beta. open beta, as the name suggests, opens it up to interested average joe candidates. each company has different selection methods, but most are by sign-up. closed betas are usually conducted by people (internal or outsourced) paid to test the games. its not as glamorous as it sounds, as these people deal with massively buggy builds and have to perform often robotic tasks while testing a game. they also have a lot of paper work to fill out, unlike open betas where you can usually post to a forum or submit a few lines and send it to the devs. these guys are similar to hardware Q&A testers. not much fun IMO. they just put new hardware through the rigors of daily applications and test compatibility with other hardware. if you are looking to get into the gaming business, try to get in on the product development side rather than the Q&A side.
Chiz