A bluff is a move in which a player tries to convince the opposition that his hand is stronger (or weaker) than it actually is. It can consist of many actions. The bluff is an attempt to get the opposition to believe something untrue to get him to act on it; either by folding or by betting large amounts against a strong hand, because he doesn't believe it to be strong. A bluff can therefore take on many forms and isn't always an attempt to get an opponent to fold.
Types of Bluff
We distinguish between pure bluffs, where a player can only win by getting the opponent to fold, and semi bluffs, where a player has a poor hand initially, but one that might develop into a winning hand later: like a flush draw.
The profitability of a semi bluff comes from the combined chances that either the opponent will fold to your bet, so your hand will develop into strong one, or the opponent will bet more chips because he does not believe your bet.