An important attribute of a successful poker player is the ability to be aggressive. Not blindly aggressive mind you, but rather to be selectively so. It's a well known and often repeated fact in poker that there are more ways to win a hand if the player is aggressive than if he is passive. If the player is passive, he either checks, and calls, or checks and folds. An aggressive player will either bet, or check-raise. If the player check-calls, he call only win if he has the best hand. If he bets out, or check-raises, then he can win by having the best hand, or forcing the other player to fold under the other two circumstances, obviously the better play most of the time.
So often a good player can pick up "tells", or overt mannerisms revealing the strength of ones hand, or covert betting patterns the opponent has made. By keeping these tells in mind, a good player can use them to his advantage. For instance, if a player knows his opponent rarely bluffs, yet will often play timidly in the face of a bet or a raise, he can use the information to play strong against the opponent with impunity.
Taking the aggressive play styling a step further, one can capitalize on "fast" play in circumstances that lend them well to such tactics. For instance, depending on position in the hand, one can raise an opponent, not because he thinks his hand is best at the moment, rather so he will get a "free" card on the next street of betting if needed. The "free card" play is most often utilized when a player has a straight draw or flush draw. This could be especially useful if the player is able to raise on the "cheaper" street so that he does not have to call a full bet on the more expensive rounds.
As with everything in poker, playing aggressively is not a style that should be employed all the time. If for no other reason, an aware opponent may pick up on a "too aggressive" style and exploit it. So as with all the other tools in a successful poker player's arsenal, the aggressive style is an important one.
To summarize, when playing poker if there is ever a question as to the best course of action in a particular situation, the more aggressive play is usually warranted. This maxim can be applied to both poker tournaments and live games.