Both throw something heavy a long distance but what separates the two?
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Imagine you have a plastic spoon and want to fling some food at at someone. You have a few options.
Hold the spoon at the handle end and then bend the scoop with your other hand, until you let go and the spoon springs back and launches your food. That would be similar to a catapult.
This time, take a rubber band and hold it stretched between two fingers. Slide the spoon handle into the gap inside the rubber band. Now rotate the spoon around and around so the rubber band starts to twist up tightly. Let go of the spoon and the unwinding rubber band will launch your food. This would be similar to an onager.
Now, take your food and place it directly into the rubber band and stretch it like a slingshot. This would be similar to a ballista.
Instead, get a pencil and lay your spoon across it to make a lever. Now drop a rock on the handle end to make the spoon launch your food. This would be similar to a trebuchet, although a real trebuchet is a bit more complicated. Rather than lying on the ground, the lever is held up in the air on a wooden frame, and rather than dropping a rock, the rock is just glued onto the handle of the spoon, with ropes holding the handle from dropping. You cut the rope to let the heavy end of the spoon fall.
Naturally, there are many other variations, so what I listed as a catapult might be called something else depending on how specific you want to be.
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