A catapult is a type of machine used as a weapon to throw rocks or other things such as hot tar, that would cause damage to something else. Often, catapults were set on higher ground or on castle towers to let them shoot farther. They shot rocks to break castle walls, or pitch or hot tar to set the target on fire.
Catapults use weights and levers to send the rocks or other things into the air. They do not throw as far as modern weapons do, and are not useful in modern warfare. Catapults were common in the past, especially the Middle Ages.
Nowadays catapults are used to study old weapons, and as experiments (for example in physics or engineering classes). Some old ones are kept in museums.
Aircraft carriers use much more powerful steam catapults to throw airplanes into the air.