A tactic is a sequence of moves that limits the opponent's options and may result in tangible gain. Tactics are usually contrasted with strategy, in which advantages take longer to be realized, and the opponent is less constrained in responding.
With this application you can solve thousands of chess tactic puzzles grouped by theme (Fork, Pin, Skewer, Double check, Discovered attack).

Fork is a tactic in which a piece attacks two or more pieces simultaneously.
Pin is a chess tactic in which a defending piece cannot move without exposing a more valuable defending piece on its other side to capture by the attacking piece.
Skewer is an attack upon two pieces in a line and is similar to a pin. A skewer is sometimes described as a "reverse pin"; the difference is that in a skewer, the more valuable piece is the one under direct attack.
Double check is a check delivered by two pieces simultaneously.
Discovered attack is a direct attack revealed when one piece moves out of the way of another.