What is a corner kick in soccer?

Answer 1

A corner kick is awarded to the attacking team if the ball completely exits the field across the goal line, was last touched by a player on the defending team, and does not enter between the goal posts and under the cross bar.

It also occurs if, during a restart, the defending team puts it directly into their own goal.

Answer 2

A corner kick occurs when defensive team causes a ball to go out of bounds over its own goal line.

To put the back in play, the ball is placed at the corner of the side of the field that ball went out of bounds. A player on the offensive team then kicks the ball back into play from that spot.

In FIFA's Laws of the Game, Law 17 describes corner kicks in more detail.