Catapults were used back in the medieval times for launching a siege on a castle and trying to take it over. They were used to take down walls so people could get in and try to push the defenders back into their castle and hopefully take over the castle. But the catapult is just a general idea, there are many different kinds of catapults and they each have different uses in war.
The main type of catapult they use is called a sling catapult, they use this a lot because it can shoot really far and it fires large boulders at the wall for a lot of damage, so basically they can get some heavy damage done on the wall from a safe distance. Those defending the castle could normally not get to catapults because to get to them they had to get through the army in front of the catapults that were defending them, so it was out of the picture to try and take them down.
Another type of catapult (kind of like a catapult it still launched large objects) was the ballista, this catapult I guess you could call it did not launch large boulders but was basically a large bow that shot large arrows but in a catapult form. This was not used too much because of trying to make the arrows that it shot, it was easier to use just a normal catapult because rocks were lying everywhere. But even though they were rare they were still used sometimes because of the accuracy they had, they could almost always hit their target.
Besides those two types of catapults they used another kind of catapult called the trebuchet catapult. This is just like the sling catapult but it was not the most accurate catapult they had. It was very unreliable for trying to hit a specific spot on the castle. So they used it just to throw rocks at the castle to try and injure the people inside the walls. If possible they may get lucky enough to hit a wall and help with trying to take it down to get inside so they can try for the victory. Even with these three kinds of catapults firing at the wall (if they have them all when doing a siege) getting in would still be rare if they won because of the defense of the castle, and people waiting to defend when the people rush through the hole in the walls of the castle.