[AaronBrown]

You should never make any poker decision based on how much you have put into the pot, that money is gone, it's not yours any more. Another common error in a tournament is to assume you're pot committed because folding would leave you with such a short stack that your chance of winning is negligible. Those last few chips are worth more than their nominal value, so you should be more willing to fold rather than less if folding leaves you short-stacked.

I also don't like your example of being committed to prevent other players from running over you in future hands. That's a terrible reason to bet. Fold, then keep folding until you get the nuts, and then let them "run over you."

I define pot committed as a situation before the last bet in which you know you will not fold before showdown. This generally means two things:

(1) The pot is large relative to the potential amount you can put in (due either to limits or stack sizes), and

(2) You have a reasonable chance of winning even if another player bets as strongly as possible.

(1) is straightforward and easy to measure. (2) can come about in different ways. A player might be so loose, or bluff so often, that you always have a reasonable chance of winning. Or, your hand might be a surprising one, so that a player might bet very confidently against you and lose. Or, you might be in a common situation in which game theory dictates always calling and taking your lumps when someone gets a good draw.

It's important to know when you're pot committed, because it affects your betting strategy. Generally speaking, it makes you more aggressive. Since you know what you're going to do regardless of future cards or bets, you have no useful information to get, nor future decisions to make. In that case, you generally want to take away other players' decisions where possible, and force them to make remaining decisions now rather than later.

This is not always true. If you are pot committed out of pure strength, you may want other players to improve so you can get more money out of them. In that case, they think they're getting more information with future cards, but they really aren't because you'll beat them whatever cards come up. Or, you may think another player either has the nuts or is bluffing, in which case she isn't getting any information in the future. But you need a reason not to force the issue when you're pot committed, strong action is usually called for.