Bluff is certainly one of the most important poker tactics. Even though you can bluff against any player, it is most useful when playing against experienced players.
That is because inexperienced players will not even notice the messages you are attempting to convey with your body language and reactions at the poker table. On the other hand, experienced players will notice everyone and everything.
For instance, in case of strong cards we can pretend having a weak hand after the flop. In this case we would call bets only after thinking a bit, to create the impression we intend to convey to our opponents. It requires careful analysis; based on our cards, we must know that our hand is better than the hands of our opponents. After the river, we can use this tactic to try and get as many chips as possible from our opponent.
A bluff can also be supplemented by a planned loss. When we have low-value cards (2 to 8) and there aren't many chips in the bank, we can call after the river even if the table has at least 2 or 3 cards of values exceeding 10. This would be followed by showing our opponents our cards, "by accident". This will convince our opponents that we can also call when we have bad cards. Based on that, they could deduce that they should raise more aggressively when they have good cards, to get more chips from us. A bluff will then allow us to make use of this when the time comes.
It is important to keep in mind that bluffing cannot be used too often. Furthermore, the more chips there are in the bank, the less we need to bluff. Since bluffing is not only about the time delays before calling or raising but also about body language, bluffing is used predominantly when playing live.