Well, you've probably figured out the number of players and the object of the game.
Many details in rules vary according to the level of play--for instance, duration of the match increases with the level of play.
I think the biggest things are these:
--Soccer is a "non-contact" sport, but that doesn't really mean no contact; it means that contact must be incidental to playing the ball. Infractions are penalized by the awarding of a penalty kick (AKA "free kick") to the team suffering the infraction.
--If the ball goes out of bounds across a *sideline* then the team which did not touch the ball last gets a "throw-in"; if it goes out across a backline, then one of two things happens. If the ball was touched out by the attacking team, then the defending team gets a goal kick, taken by the goalkeeper. If the defending team touches the ball out, then the attacking team gets a corner kick--a free kick from the corner of the field, which often provides an excellent chance to score.
--The off-side rule often confuses people. Basically, the ball is supposed to precede attackers when they try to get past defenders to reach the goal.
If an attacking player is closer to the opposing goal than all but the keeper, he is in an off-side position. Off-sides will then be called if he receives a pass AND he was off-side *at the time the ball was kicked to him.*
Many goals have been negated because an attacker was called off-side. Sometimes, by contrast, players successfully "time" their rushes up-field to pass that crucial second-last defender a split-second *after* the ball was kicked; not infrequently, some spectators think that off-sides should have been called in such cases, either because they don't understand the rule fully, or because it appeared to them the player was a bit too early. And sometimes, defenders will deliberately move to place attackers in an off-side position, though that's a bit dangerous because they're relying on the referee to make the call.