Reading Other Players

If you've determined that a player is a rock, and you have JJ preflop in a game of Texas Holdem with that person, you can alter your decision making process with that player. Suppose you raise preflop with JJ, and the rock raises you all-in. You might want to fold.

On the other hand, if you're playing with a maniac who has been ramming and jamming with A nothing offsuit all night, you might want to call that all-in. (Circumstances might vary, but it's clear that getting at least a peripheral understanding of your opponents' tendencies can affect your win-loss rate.)