The King is the most important piece, but it is one of the weakest. When the King is captured, the game is over.
The King may capture an opponent's piece when it is not defended, or even if it is threatening him ("in check").
The King can only move one square in any direction: up, down, to the sides, and diagonally.
Forbidden Squares for the King
The King may not capture defended pieces, because he could be captured.
The King may never move himself into "check" (where he could be captured).
Castling and Check - Checkmate.
Ajedrez - Chess