There's basically three types of crossbows:
Light crossbows can be loaded by hand, which makes them similar in use to bows, with the added benefit of not having to manually keep the string pulled back while aiming.
Heavy crossbows have a much higher draw weight and can only be pulled back by turning a winch. Thus they take much longer to load than a light crossbow, but have far more power, easily capable of punching through steel plate. They're meant to be used from behind cover rather than on the battlefield, due to the long reload.
Repeater crossbows have a different kind of winch that when turned pulls back and releases the string repeatedly. They have a magazine of arrows mounted on top, so every time the string is pulled back a new bolt automatically falls in its place, ready to be fired. That way you can aim the crossbow and just turn the winch to automatically draw, shoot, draw, shoot, draw, shoot several bolts in a row. The power is comparable to that of a light crossbow.
I hope that helps :)