The rules of football are officially referred to as the "Laws of the Game". The Laws of the Game are described very precisely (approximately 150 pages) on the FIFA website here.
There are 17 laws in total, each one briefly summarised below.
Field of Play. The game can be played on either natural or artificial surfaces, the surface must be green and rectangular in shape. The two long sides of the rectangle are called touch lines and the two shorter sides are called goal lines. The field is divided in half by the halfway line.
Ball. Must be spherical, made of leather (or similar) 68-70 cm in circumference and of a certain pressure.
Number of Players. Two teams of no more than 11 players (one of which is the goalkeeper). A game cannot start if either team has less than 7 players.
Equipment. Players must wear a jersey, shorts, stockings, shinguards and footwear.
Referee. The referee ensures the Laws of the Game are respected and upheld.
Assistant Referees. There may be at most 2 assistant referees.
Duration of the Match. The game is played in 2 halves consisting of 45 minutes each. The half time interval must not exceed more than 15 minutes. At the discretion of the referee more time is allowed to compensate for any stoppage during play e.g. Due to substitutions or care and attention of injured players.
Start and Restart of Play. A kick-off starts play at the start of the match or after a goal. A kick-off involves one player kicking the ball, from stationary, forward from the centre spot. All players must be in their own half prior to kick-off. A coin is tossed pre-game, the team which loses the toss are awarded the kick-off to start the game whilst the team that win the toss are allowed to choose which direction they want to play. After half time the teams switch direction and the other team will kick-off. After a goal is scored, the team which conceded the goal will kick-off to restart play.
Ball in and Out of Play. The ball is out of play once a goal has been scored or when the referee has stopped the game. The ball is in play at all other times.
Method of Scoring. The ball crosses the goal line inside the goal mouth.
Offside. It is an offence for a player to be in contact with the ball when they are closer to the opponents' goal than both the ball and the second-last opponent. The offside rule exists to ensure there are always opponents (generally the goal keeper and a defender) between a player receiving the ball and the goal. Without the offside rule, play can become boring with repeated long balls being kicked to a player stood next to the goalkeeper for an easy goal.
Fouls/Misconduct. These are many and varied, broadly speaking it is an offence to use excessive force whilst playing the game either deliberately or undeliberately or to handle the ball (unless you are a goal keeper). The referee may show the yellow card to caution players for less serious offences and the red card for more serious offences resulting in the player being sent off. Two yellow cards are equivalent to one red card.
Free Kicks. Are given by the referee for fouls and misconduct. A free kick can either be direct or indirect. A goal can be scored directly from a direct free kick. A goal can only be scored from an indirect free kick if it touches at least one other player first. The free kick must be taken from a stationary position with that position varying depending on whether the free kick was given inside or outside the goal area and whether it's direct or indirect. The opposing team must be a minimum of 9.15 m from the ball when the free kick is taken.
Throw-in. Used to restart play after the whole of the ball has crossed the touch line.
Goal kick. Used to restart play after a goal has been scored.
Corner Kick. Is given when the whole of the ball crosses the goal line and was last touched by a member of the defending team (and no goal was scored). A corner kick is taken from inside the corner arc closest to the point where the ball crosses the goal line. The defending team must be at least 9.15 m from the ball when the corner kick is taken.
