The answer given by Joe Landenneau is absolutely the correct answer.
Also,on a tackle from behind can be a legal tackle. It is not automatically a foul. If the tackle is an illegal tackle from behind, the officials do not always red card the tackler but he does always get yellow carded.
Another call that is missed is a legal slide tackle and the player losing the ball to the tackler continue on and trips over the tacklers legs. I do not consider that as a foul but I have seen it called several times.
