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 (Zugzwang, Quiet move, Zwischenzug, Defense, Hanging piece, Trapped piece).

Zugzwang is a situation found in chess wherein one player is put at a disadvantage because of their obligation to make a move; a player is said to be "in zugzwang" when any legal move will worsen their position.
Quiet move is a move that does neither make a check or capture, nor an immediate threat to capture, but does prepare a more hidden unavoidable threat for a later move.
Zwischenzug is a chess tactic in which a player, instead of playing the expected move (commonly a recapture), first interposes another move posing an immediate threat that the opponent must answer, and only then plays the expected move.
Defense: to play defense, or to defend against an opponent's threat.
Hanging piece is an attacked piece not defended by own man exposed to capture.
Trapped piece is a piece that finds itself with either no moves at all or at least no moves that avoid the loss of material.