Ooops! I meant to ask what a yellow CARD is for, not a yellow FLAG! *blush*?

Answer 1

The yellow card is a penalty for committing a brutal or tactical foul. If a player gets 2 yellow cards he is sent of the field so a card is a warning by the referee for the player.

Answer 2

Yellow and red cards were first introduced in the game by British referee Ken Aston and its first major use was in the 1970 World Cup:
The cards are shown to the players who have committed a foul. The color of the card depends on the severity of the foul.
A yellow card is shown by the referee to indicate that a player has been officially cautioned.
A red card is shown by the referee to indicate that a player has been officially send-off or expelled from the game, and must leave the field inmediatly and won't be able to play the rest of the game.

Receving a yellow card caution
There are seven different offences that can get you a yellow card:
# Anything that can be deemed as unsporting behaviour
# Dissent by word or action
# Persistent infringement of the laws, for example a series of fouls
# Delaying the restart of play
# Not retreating the full ten yards at a free-kick or corner
# Entering or re-entering the field without the referee's permission
# Deliberately leaving the field without the referee's permission

Receving a instant red card send-off
There are seven offences you can be shown a straight red card for:
# Serious foul play.This includes a tackle that endangers the safety of an opponent.
# Violent conduct such as throwing a punch
# Spitting at an opponent or another person
# A player other than the goalkeeper denies an obvious goalscoring opportunity by handling the ball
# A goalkeeper would be similarly punished if they intentionally handle the ball outside their goal area
# Committing an offence that denies an opponent an obvious goal-scoring opportunity (informally known as a professional foul)
# Using offensive, insulting or abusive language or gestures

Receiving Second Yellow card
If the referee shows a player a second yellow card in the game, the referee will show the second yellow card before holding up the red card, meaning that the player is sent off or expelled from the game.

Suspensions (Red card)
Rule: A red card implies 1 game suspension.
If a player gets a red card, in the final game, he will carry over the punishment to the next international game of his team, when he plays.

Carry on the cautions (Yellow card)
The exact punishments are determined by tournament or competition rules (not by the Laws of the Game). The cautions are accumulate for a certain phase of a the world cup tournament.
Rules:
* A yellow card is carry over, and 2 yellow cards equal a red and a 1 game suspension.
* A yellow card is carry on during the first and second phase of the tournament.
* For the quaterfinals is a clean start for all the players, except for a red card.
* Begining the quaterfinals the yellow cards are carry on, until the end of the tournament. And at the end of the tournament all the yellows cards are reset, except:
* If a player gets a second yellow card, in the final game in which his team participated, he will carry over the punishment to the next international game of his team, when he plays (he has to be in the team roster).
* If a player gets a red card, in the final game in which his team participated, he will carry over the punishment to the next international game of his team, when he plays (he has to be in the team roster).

Answer 3

yellow card is for strong but not brutal physical contact. sometimes for a handball. or you could get a yellow card by yelling at the ref

Answer 4

i yellow card is for doing a fould to someone on accident

Answer 5

for many things, a fowl, cussing calling a ball when its in the air, jersey pull things like that