At the senior level, footy is played over four quarters of 20 minutes each, with time being added on for delay of game, player injury, recovery of the ball as a result of scoring and out of bounds. This time on will be signalled by the central umpires but recorded and added to normal time by the timekeepers.
The game is started at each quarter and restarted after each goal by one of the central umpires bouncing or throwing the ball up in the centre circle . The ruckmen must be within the larger circle at the time the ball is bounced or thrown up. At this time, only four players from each team are permitted in the centre square and no player may be in the centre circle until the ball is bounced . Infringements to these rules result in a free kick to the opposition from the centre circle.
A goal is scored when the ball is kicked over the goal line by a player of the attacking team without the ball touching a player or goal post. To score, the ball must completely cross the goal line.
A behind ( 1 point ) scored when the ball passes over the goal line in any way other than just described , touches or passes over a goal post or passes over a behind line without touching or passing over a behind post. A defending player cannot score a goal for the opposition. If he kicks or takes the ball over either the goal or behind the line, a behind is scored for the attacking team.
After a behind has been scored, any player from the defending team kicks the ball back into play from within the kick-off lines in front of goal . The player kicking the football into play may provided he has kicked the football clear from his hands ., regain possession and play on from the goal square. This is rarely seen however most kick-offs are kicked along out of the scoring area with all other players being at least 5 metres from the kick-off line when the ball is kicked.
To go out of bounds the ball must completely cross the boundary line. If the ball goes out of bounds other than on the full from a kick or directly from a kick-off without being touched by an player , a boundary umpire throws the ball back over his head towards the centre of the ground into play. If the ball goes out of bounds on the full from a kick, directly from kick-off without being touched by any player whether on the full or not or from being deliberately knocked or carried over the boundary line, a free kick is awarded to the nearest player from the opposite team.
Reference: Australian football steps to success : By Andre McLeod and Trevor D . Jacques