So, a QA tester is someone who wants to be a game designer but hasn't been able to get that job yet, so they're willing to actually test the games instead.
Well, that's a common story for a game tester, but really game testing, also known as QA or quality insurance, is the act of playing the game as if you're the end user and trying to make sure there are no bugs or problems with the game that may it un-enjoyable, hard to play, or just darn broken. So, actually I came out of QA. After college I started my own game company and then when that didn't work out I got in the business to do QA but to figure out how games were made. The thing about being in QA is that you get exposed to all aspects of production of games.
You learn about art, you learn about code, you learn about game design, and often if you're diligent, and you're good to work with, and you have some amount of humility you'll develop a rapport with the developers in the organization and get an opportunity to move up one day. So, QA can be a great way to enter the gaming industry, but I also know guys that have made a career out of QA. There are QA directors now, QA manager, and their an integral part of making a successful game.