Warwick Castle's Trebuchet, firing, here Apologies for the windy day making the sound quality poor!
A trebuchet (pronounced, tre-bew-chay) is a seige catapult. The one at Warwick Castle was made out of the finest English Oak. Now this isn't patriotism on my part, but is due to the genetics and environment in which English oaks grow; they form a dense, slow growing, heavy and therefore very strong wood. The trebuchet pictured above is 22 tons of solid oak. It has an ash arm (ash being lighter and much more flexible which allows more spring and therefore greater distance when firing) The counterweight is 6.5 tons.
If you look at the trebuchet closely you will see some wheels in the centre that look like something a hamster would use for exercise. A team of 4 men would crawl inside the wheel arrangement and walk around, winding a rope around a centre cog. This would pull the arm of the trebuchet down to the ground where it was fixed to prepare for firing. This could also be done by horses pulling the rope away from the trebuchet. The counter weight rises to the top of the frame.
(below, French Trebuchets; photo courtesy of Wikipedia)
A stone boulder is loaded into the sling. This could also be wrapped in burning cloth if a fire ball was required. (There's a lot of wood inside a castle) One trebuchet against a Castle wall wouldn't have much immediate effect but they were used in batteries of up to 40 or 50. As well as boulders, it was important to demoralize the enemy to prevent long seiges, so rotten meat, urine, and toilet waste, entrails, and the heads of prisoners were also fired. Polluting the drinking water and spreading disease were also very important weapons of war.
In fact, spreading disease and mayhem were more important than battering holes in castle walls. It was necessary to gain entry, but a castle was a useful thing to capture, and you wouldn't necessarily want one full of holes! Imagine the horror of rotting carcasses hitting the ground around you at over 150 mph.