A salad spinner is a small kitchen appliance that normally uses a pumping action to quickly rotate an internal basket in which washed salad leaves are placed. The centrifugal force quickly removes any excess water without damage to the contents.
Some models, instead of the spinning top-type pump action already described, have a folding levered handle (or just a knob) which is attached internally to a gearing system. Thus turning the handle at 1 revolution per second will produce (for example, I'm just guessing) an internal speed of the basket of 5 r.p.s. The water is ejected via centrifugal force through the holes in exactly the same way as above.
A salad spinner is great for washing and drying herbs - we use ours more for this than we do for drying salad leaves.