A bluff is a bet or raise made by a player with a hand they believe is not the best hand, nor is it likely to become the best hand. In short, it's a bet or raise done with "no hand and no draw", where the player is merely representing (attempting to give other players the impression) that they have the best hand.
Note that making a bet or raise without the best hand, but with a draw, is called semibluffing. And note that betting with a mediocre hand that may or may not be best is not bluffing: it may be value betting or "finding out where you are", but bluffing, by definition, is betting when you have nothing, and are virutally certain to be behind one or more other players in the pot.
Bluffing is one of the most popular moves in poker, but is probably done far too often in most small stakes no-limit games, and is not often performed well. In order to be effective, bluffs must:
- be done when other players have either no hand (high card) or weak hands that they will not be able to call with. Players with two pair or stronger hands often feel forced to call: if you put a player on a hand of two pair or better, you'd often be better served to fold than to try to bluff them
- be of an amount that is too high for opponents to call with their weak hands. A bluff that is too small may result in a crying call from an opponent instead of a fold.
- not be so high an amount that it "seems supiciously like a bluff". Overbetting the pot is a supposedly common tactic among bluffers, and can cause adventurous opponents to call.
- be done against fewer opponents rather than more. It's possible to bluff many players at once, but it is much more difficult, because it's that much more likely that one of your opponents has a hand which they would call with
- not be done against a calling station. If a player will call you with anything, even Ace-high, you cannot bluff them. Don't try. Instead, win their money by using value bets.
Bluffing is very difficult in fixed-limit games, because of the second guideline above: because of the fixed bet size, it is often impossible for a player to make a bet too large for someone to call. Especially in low-limit games where the pots can get very large, even good players will often make a crying call when faced with a bluff due to the enormous pot odds they are getting.
Some players say bluffing is impossible in fixed-limit games (or in low-limit games), but that is simply not true. Bluffing can be done against small fields, in position, or when you have a clear read that players are on draws. Bluffs can even be made against strong players if you have built up a table image of being tight and conservative.
Bluffing is much more common, and much more effective, in no-limit games, where the fact that a player can control the size of their bet makes it possible to follow all the guidelines and bluff more frequently and effectively.
Bluffing and game theory
Basic game theory says that you should bluff often enough so even if your opponent(s) always call or always fold, you still come out ahead.