Pawn promotion is a special move of pawns in Chess. It happens when a pawn reaches the back rank of the opponent, i.e. the opposite side of the board, and then it is replaced by any piece which that side had at the beginning of the game except pawn and king: queen, rook, bishop or knight. The choice is up to the player making that move, and is not restricted to the captured pieces; this means that a player can have two queens, three rooks etc. at once. The most common choice is naturally the queen. Promoting to a queen is called queening, and to an other piece is called underpromotion.
White pawn before and after promotion
Black pawn before and after promotion
Further example for pawn promotion:
Pawn promotion can also be a capturing move. Here the white pawn captures the rook, promotes to a bishop (underpromotion), and gives check to the king – all in one single move.
Pawn promotion occurs mostly in endgames (i.e. when only few pieces remained on the board). There passed pawns are very valuable, i.e. pawns without opposing pawn stopping them from promotion (either by blocking it, or by threatening to capture it).
The chess variants have usually similar pawn promotion rules as chess, but the promotion fields and the piece choice may differ. Nevertheless some general rules can be given (but there are exceptions):
Example of unusual promotion rules: