In divisions U12 and up, you should have an Assitant Referee at each touchline. They have flag and have specific responsibilities. Because the referee is busy keeping an eye on everything on the field, the assistants help keep an eye on the game also, but from a different point of view. They keep an eye on the touchline and will signal with the flag if the ball crosses the touchline COMPLETELY (discussed later).
They also are responsible for keeping an eye on the offside situation (discussed later). They watch the field and if the referee is in a position were he/she couldn't see a foul committed and the assistant referee does, the assitant will signal with a flag.
Assistant Referees do more in a game than that, and are indispensable to a center referee. Good Assistant referees (ARs) are a dream to have for a center referee and often teams referee together as much as possible.
They are to be given the same respect as a referee. Don't start talking to the AR while the game is going. They need to concentrate on the game and should not answer questions about referee calls.