Catapults are siege engines that use an arm to hurl a projectile a great distance. Technically any machine that hurls an object can be considered a catapult, but the term is generally understood to mean a specific type of medieval siege weapon. Originally, "catapult" referred to a dart-thrower, while "ballista" referred to a stone-thrower, but ironically over the years, the two terms eventually switched meanings.
Catapults were usually assembled at the site of a siege, and an army carried few or no pieces of it with them because the needed wood was usually easily available on site. The Greeks came up with the earliest form of a catapult, which was similar to a very large crossbow, but adapted to have a sling that would allow all sorts of objects to be hurled, including pots of the infamous Greek fire.
The mangonel followed. On a mangonel, the bottom end of the throwing arm and the inner ends of both ballista arms are inserted into rope or fibers that are twisted, providing a stronger store of energy because of the torque. Torsional ballistas had greater power. Most mangonels have an arm with a bucket, cup, or most often a sling to hold the projectile at one end.
Finally, the last type of catapult, which was also the king of all Medieval siege weapons, is a trebuchet, which used gravity or traction, rather than tension or torsion, to propel the throwing arm. A falling counterweight, would pull down the bottom end of the arm and the projectile is thrown from a sling attached to a rope hanging from the top end of the arm. In layman’s terms, the weapon is similar to a giant sling attached to a giant see-saw. The counterweight was usually much heavier than the projectile, to assure maximum distance and force.
Through out Medieval times, catapults and related siege machines were the first weapons used for biological warfare. The carcasses of diseased animals and those who had perished from the Black Death or other diseases were loaded onto the catapult and then thrown over the castle's walls to infect those barricaded inside. There are other recorded instances of large numbers of beehives being catapulted over castle walls.
The catapult was a necessary weapon since a normal military confrontation in Medieval times was for one force to hole up in a castle, and another to lay siege to it. Without siege weapons, the attacking army would have to starve the people out by blocking supplies, which could take months to years. Siege weapons allowed a more proactive approach to the matter, and the basic catapult was much easier to assemble than a trebuchet, though even the mighty trebuchet, when all is said and done, is a catapult. These are the weapons of seige that Medieval armies relied on, and deserve their recognized place in history.