Law 10 - Method of Scoring

Except as otherwise provided by these Laws, a goal is scored when the whole of the ball has passed over the goal-line, between the goal-posts and under the cross-bar, provided it has not been thrown, carried or intentionally propelled by hand or arm, by a player of the attacking side, except in the case of a goalkeeper, who is within his own penalty-area.

The team scoring the greater number of goals during a game shall be the winner; if no goals or an equal number of goals are scored, the game shall be termed a "draw".

Decisions of the International F.A. Board

Law 10 defines the only method according to which a match is won or drawn; no variation whatsoever can be authorized.
A goal cannot in any case be allowed if the ball has been prevented by some outside agent from passing over the goal-line. If this happens in the normal course of play, other than at the taking of a PK: the game must be stopped and drop ball.
If, when the ball is going into the goal, a spectator enters the field before it passes wholly over the goal-line and tries to prevent a score, a goal shall be allowed if the ball goes into the goal unless the spectator has made contact with the ball or has interfered with play, in which case the referee shall stop the game and restart it by drop ball.