A situation in which a referee or linesman drops the puck between two opposing players to start or resume the play of action at a designated spot on the ice. A faceoff begins when the official designates the location of the puck drop then the two opposing players meet at the spot and the faceoff ends when the official drops the puck to start the play.
Players can be ejected from the faceoff for a number of reasons including if the players aren't centered correctly, are encroaching on the face off spot, make contact with the opponent, have placed their stick on the ice incorrectly and/or a player is in the offside position.
Faceoffs are defined under Rule 76.1 in the Official NHL Rulebook.
Generally, the two opposing players taking the faceoff will be the centers on the line but sometimes a winger will take the faceoff if he is known to have a higher faceoff percentage wins.
Faceoffs are an important aspect of the game as they can determine the control of play. For example, there is a great advantage for teams being able to win a faceoff and maintain control of the puck in the offensive zone as it allows them to make a potential scoring play. If they have a weakness in the faceoff, the defensive team is likely to win control of the faceoff and get the puck out of their zone ending the other teams attack.