The main difference between Trebuchet and Ballista is that the Trebuchet is a type of siege engine which uses a swinging arm to throw a missile at the enemy and Ballista is a ancient missile weapon
A trebuchet (French trébuchet) is a type of catapult that uses a swinging arm to throw a projectile. It was a common powerful siege engine until the advent of gunpowder.
There are two main types of trebuchets. The first is the traction trebuchet, or mangonel, which uses manpower to swing the arm. It first appeared in China in the 4th century BC. Carried westward by the Avars, the technology was adopted by the Byzantines in the late 6th century AD and by their neighbors in the following centuries.
The later, and often larger, counterweight trebuchet, also known as the counterpoise trebuchet, uses a counterweight to swing the arm. It appeared in both Christian and Muslim lands around the Mediterranean in the 12th century, and made its way back to China via Mongol conquests in the 13th century.
The ballista (Latin, from Greek βαλλίστρα ballistra and that from βάλλω ballō, “throw”), plural ballistae, sometimes called bolt thrower, was an ancient missile weapon that launched either bolts or stones at a distant target.
Developed from earlier Greek weapons, it relied upon different mechanics, using two levers with torsion springs instead of a tension prod (the bow part of a modern crossbow). The springs consisting of several loops of twisted skeins. Early versions projected heavy darts or spherical stone projectiles of various sizes for siege warfare. It developed into a smaller precision weapon, the scorpio, and possibly the polybolos.
A medieval siege engine consisting of a large pivoting arm heavily weighted on one end.
“Medieval trebuchets are said to have been capable of launching 90kg projectiles over distances of more than 300 meters, making them more powerful than most pre-modern types of catapult.”
A torture device for dunking suspected witches by means of a chair attached to the end of a long pole.
An ancient military engine, in the form of a crossbow, used for hurling large missiles.