A change of possession in football is when possession of the football is switched from one team to the other. A change of possession can occur in five different ways: turnovers, missed field goals, punts, kickoffs, and the coin toss.
Turnovers include fumbles, interceptions, and turnovers on downs, all resulting in one team losing the football and possession being turned over to the opposing team (assuming the team that fumbled does not recover the fumble).
A missed field goal results in the opposing team receiving the ball at the spot of the kick. Since field goals are taken on fourth down, if the kicker misses, the offense loses the ball since they did not convert for a touchdown or a first down.
Punts and kickoffs differ from missed field goals or turnovers because they are voluntary changes of possession. Both involve the punter kicking the ball to the other team so that they can start their drive down the field. The kickoff occurs at the beginning of both halves, while punts occur after failed attempts to drive down the field for a touchdown.
The coin toss at the beginning of the game determines who has possession of the ball first. They can choose to receive the ball and gain the first possession of the game, or defer to the other team and have the first possession in the second half of the game.