Reckless challenges (tackles) can get you a yellow card, along with a few other things. If you get two in a match, you're out and your team continues on a man down.
If it's especially egregious like a punch, spikes up or a kick to the face, then it's a red and you're off immediately.
The situation that comes up frequently is denial of a clear goal scoring opportunity, like if somebody was on a clear run to the goal and rather try and take the ball, you just bowl them over.