Sorry, I figured most would know what a trebuchet is.
Weiser is right, it's a medeival catapult type seige weapon used for slinging rocks, fstuff on fire, diseased cattle, etc.
It works by using a sling and counterweight and uses the same principle as a playground see-saw.
The falling weight causes the arm to move up, which make the sling fly out in an ark. When the sling gets to a certain point, part of the rope/string comes off of of pin which causes the part of the sling holding the projectile to come open and let go of it.
Here's some pictures:
