Tactic in chess usually involves a succession of moves which are based on forcing the opponent into making moves that disadvantage him. The purpose behind the tactical procedure is to gain material or positional advantage or to save the game. This combination of moves is usually spectacular because at a less careful analysis of the board it is hard to spot the objectives behind it.
In order to control the middle game you have to be familiar with elements of tactic and strategy (strategy involves establishing the purpose of the game and making the general plan by analyzing the position and is a more abstract notion than tactic). Tactic and strategy must be combined together in order to help the player to determine what must be done and how that will be done.
There is a common misconception that you can only learn strategy and that tactic is a matter of talent. That is not true. You can learn tactic just like you would learn strategy. All the way from the beginning of the history of modern chess there have been noticed recurring positions that kept appearing in the game. This positions lead to the theory of the end game and that of tactic. It's easy to imagine such typical positions in the end game (where because of the limited number of pieces on the board the same ending can occur again and again). Well, pretty much the same thing is happening in the central game too (although there are a great number of pieces on the chess board, in this part of the game, only a few actively participate at the tactical operation; the great number of pieces on the board also implies that the typical positions in the central game have a more general aspect then the position from the ending).
You may have heard about the notion of combination. A combination is a particular case of a tactical maneuver in which a sacrifice is being done. One of the great chess players of the world, Botvinnik, gave a good definition of this notion: " The combination is a forced version of sacrifice". Botvinnik states that the forced tactical maneuver, without a sacrifice, must not be mistaken with the combination. So, in the end, you could say that the thing that characterizes the combination is the sacrifice.
Now that we made this short introduction to tactic it's time to move on to introducing and explaining each tactical procedure.