In soccer there are lots of fouls that the refs call.  If a player is kicking or attempting to kick an opponent he or she will get a card.  If the player accidentally kicks the opponent while attacking the ball then he or she will just be warned or they will get a yellow card.  If it is on purpose and they were acting careless and reckless and using excessive force the player will be given a red card.  A player can also get a card if they’re tripping an opponent, charging them, purposely touching the ball to prevent the other team from scoring, and holding or pulling on an opponent.  It all depends on how serious and forceful the foul was on whether they’d get a red card or a yellow card.  If fans don’t agree with the refs they have no say.  The ref gets to choose if it was a foul or not.