Checkmate (frequently shortened to mate) is a situation in chess in which one player's king is in check and there is no way to remove that threat; it is a check from which there is no legal move to remove check through either capturing the piece checking the king, moving between the piece delivering check and the king, or moving the king itself. In checkmate, the king is never actually captured—the game ends as soon as the king is checkmated. A player who is checkmated loses the game. Delivering checkmate is what all players strive for in chess, although not all games end in checkmate. Most players resign before being checkmated, or the game may end in a draw.
A minimum amount of force is required to force a checkmate. If an endgame is reached where one player has only his lone king remaining on the board, the other player will require at least a queen, or a rook, or 2 bishops, or a bishop and a knight, or a pawn which can be promoted. Checkmate cannot be forced with a knight, or with 2 knights (therefore it may be said that bishops are better than knights in endgames, and retaining a pair of bishops is a long term strategic advantage).
It may however be possible to force checkmate with less than that mininum required force if the player being checkmated still has pieces on the board which can hinder him, for example, a smothered mate delivered by a lone knight, or a Philidor type mate delivered by 2 knights.
In an endgame scenario certain checkmates are easier to achieve than others.
The Lawnmower mate is the simplest, where a pair of rooks, or a rook and a queen (or 2 queens) combine to cut off one rank or file at a time, driving the enemy king to the edge of the board until it is mated there.
The lone king v king and queen mate is delivered on the edge of the board, but the king must assist the queen. Likewise in the "box mate" of king v king and rook, the king must assist.
The king v king and 2 bishop mate is like the lawnmower mate but using diagonals rather than ranks or files, mating the king in the corner.
The king v king, bishop and knight mate is the trickiest, but can be done within the 50 move rule. The king must be mated one of the 2 corners which is the colour the bishop is on. If you have a light squared bishop you need to drive the enemy king to a light corner, using the knight, bishop and king to create a "wall" of controlled squares which the mated king may not pass in to.
Should a position arise where neither player has enough pieces on the board with which he can force checkmate, the game is a draw by insufficient material.