The king moves one square in any direction: forwards, backwards, sideways or diagonally. Lose it and you lose the game — capturing the king is never actually played; the game ends one move earlier, at checkmate.
The two kings have different characters written on them: the stronger player (or Sente) traditionally takes the one marked 王将 and the other takes 玉将. They move identically.
The king can't walk into check
A king may never move to a square an enemy piece attacks. Here Gote's rook on i6 controls the whole of rank 6, so the three squares above the king are off-limits.
Try it
Try it: Move the king to every square it may go to. The board resets after each move.