You can't strike out on foul balls. So you can hit 2 foul and get 2 strikes, but after that you're safe. Hitting lots of foul balls can also be a strategy to wear down the pitcher- to tire him out, to get him to try a different pitch that the hitter likes better, etc.

It should be noted if you hit the ball into foul territory and the catcher/infielder catches it, that's is a foul out. But strike 3 can't be foul.