Bluffing is an art form, which is a vital weapon in winning pots, as you do not have to have the best hand to win. There are 2 types of bluffs. These are the pure bluff and the semi bluff.
The Pure Bluff, or simply just a "bluff", is where you put money into the pot (whether that be bet or raise / re-raise), when you have the weakest hand. This is a powerful tool in your poker play because it allows you to win the pot, when ordinarily; you would lose as you have the weakest hand.
Bluffing is telling a story that is believable. By making a bluff, you are telling a story, just like any action you make at the poker table, whether that be any betting action you make (check, call, raise, fold) or the way you act (body language). Bluffing is telling a story that you represent strength. By bluffing, you are trying to show your opponents that you have a strong hand.
In order to successfully implement a bluff, you need to know when to bluff. Like anything in poker, you need to take in the whole context of the game. Not just the short run but the long run. Aspects of the game such as the number of opponents at the table, the opponent's tendencies, playing styles, hand ranges, number of players to the flop, position, etc. All factors need to be assessed and a bluff should only be made when taking in all the factors, that the bluff will be profitable in the long run.
Something to note on that last sentence was "profitable in the long run". In factoring all aspects of the game, the context, or situation; it might be worth making a bluff knowing that you will get called. By making this play, it allows your opponents to see that you will make bluffs to try and win the pot. In the long run, this may (and often is) a profitable play as your opponents are likely to think you play a bit loose and make bluffs so they will more likely pay you off when you have a strong hand.
Semi Bluffing is very similar to the pure bluff. A semi bluff is when you make a betting action, with hand that is not the strongest, or you believe is not the strongest, but has the potential to improve into a winning hand. The classic example of a semi bluff is when you're on a flush draw or straight draw. If you're on a flush draw after the flop, you have approximately a 35% chance of hitting a flush on either the turn or the river. About 20% on the turn and 17% on the river (approx for heads up situation at the flop) to making the flush. It may be worth firing out a bet in these situations if you can get the opponent to fold. Even if they do call, a 35% chance of hitting "the nuts" is not a too bad situation to be in. You will hit the flush 1/3 of the times and if you make the right bets against the right opponent, you will get paid off.
Bluffing is a very handy strategy to have in your poker play; however, it is vital to only use it in situations which it will be profitable in the long run.