Most actions on a soccer field are fouls only by degree, and become fouls only if done in an unfair manner. Players often bump into each other while running, or push past each while each is trying to avoid a collision. These actions are just part of the game, and most bodily contact is quite incidental to the players’ attempts to win the ball.
Sometimes, though, players will exceed the bounds of fair play, either through enthusiasm or frustration, or simply by overestimating their body’s ability to do what their brains are telling them to do. It is up to the referee to decide when those actions will exceed the bounds of fair play.
Soccer is a contact sport. This means that the players can use their bodies and muscles to try to win the ball. On the other hand, soccer is not ice hockey; and unlike some other sports, soccer players are not supposed to crash into each other, or body-check their opponents. For the referee trying to keep things under control, the important thing will be the force of the contact, as well as the targeted area of the opponent’s body. Also, once the goalkeeper collects the ball, opposing players may not lawfully try to play it.
Fans and players often hear about “shoulder-to-shoulder” charges, and these are the charges that most coaches try to teach. Unless undertaken with a running start, most shoulder charges will be allowed, but this is not, strictly speaking, a requirement of a fair charge. Owing to human anatomy, though, most fair charges will come in the general direction of the shoulder area, not by use of the hips–and never directed toward back or the spine. If performed with clearly undue force, particularly toward the small of the back, a charge may be deemed reckless or worse, and punished with a caution and a yellow card–or, in extreme cases, with a red card and send-off.