Soccer has three officials in the most standard configuration of a Center and two Linesmen. While the Center Ref has veto power, the Linesmen can call fouls. How much they call is usually dependent on their experience level.
Another configuration uses two center refs. While used at a lot of high school games, and for recreational leagues, it's frowned upon by the U.S.S.F., or at least was, when I was doing this a lot 15 years ago.
A lot of times, minor fouls are not called as they would adversely affect the tempo of the game. Conversely, some times it's appropriate to call everything to keep the game from getting out of hand. Unlike football or basketball, with soccer, unless it's a direct free kick foul in or near the box (in the box results in a penalty kick), the foul penalty usually doesn't have much impact on the game.
