Zwischenzug vs zugzwang: unrelated, despite the names
They sound almost identical, they are both German, and they have nothing whatsoever to do with each other.
The difference in one sentence
Zwischenzug means in-between move: a move inserted in the middle of an exchange both sides were treating as forced, usually a check, before completing the expected recapture. Zugzwang means move compulsion: a position where you would be fine if you could pass, but every legal move makes things worse.
One is a tactic you play; the other is a property a position has. A zwischenzug is an action -- you find it, you play it, you gain a tempo or a piece. Zugzwang is a state of affairs you steer the opponent into, and the move that puts them there is often a move that does nothing at all.
They also live in different parts of the game. Zwischenzug is a middlegame phenomenon, because it needs an exchange with checks available. Zugzwang is overwhelmingly an endgame one, because it needs the opponent to have run out of harmless moves, which almost never happens with a full board.
The shared root is only the word Zug, meaning move. Zwischen means between; Zwang means compulsion.
One of each
In-Between Moves
Position 1
White to move. Find the best move.
Show the answer
37...Rxc4 38.Nxe6+ fxe6 39.Rxc4
The line wins a rook. The expected recapture is not forced, and inserting a more forcing move first changes what the whole exchange is worth.
The position comes from an endgame of a real game, rated 874 for difficulty. The answer reaches a winning advantage.
Running Out of Moves
Position 2
Black to move. Find the best move.
Show the answer
39.Kd2 39...Kd4 40.Kc2 40...Ke3
The line comes out level on material and ahead on position. Every legal move concedes something, so the winning method is to hand the obligation to move back to the opponent.
The position comes from an endgame of a real game, rated 421 for difficulty. The answer reaches a clear advantage.
Questions people ask
What is the difference between zwischenzug and zugzwang?
A zwischenzug is an in-between move inserted into an apparently forced sequence, normally a check played before a recapture. Zugzwang is a position where a player is obliged to move and every move worsens their position. The two are unrelated ideas that happen to share a German root.
Is zwischenzug the same as intermezzo?
Yes. Intermezzo is the Italian term for the identical idea, and English chess writing uses both interchangeably.
Where does zugzwang actually occur?
Almost always in endgames, especially king and pawn endings, because it requires the side to move to have no harmless moves left. With a full board there is nearly always something neutral available.