A crown is a small knob usually located on the side or top of the case that connects to the movement. The crown can vary from a simple gasket to an intricate system of gaskets and screws. In a mechanical movement, a crown winds the watch and sets time and date. In modern watches, the crown can serve a number of functions from setting the time, date, and activating other complications to a simple act of winding a watch.
Triplelock
Triplelock crown is a Rolex crown that has three seals to prevent water and debris from entering the watch. Watches that are outfitted with a triplelock crown include: Rolex Cosmograph Daytona, Rolex GMT-Master II, Rolex Yacht-Master, Rolex Deepsea, and Rolex Submariner Date.
Screwdown
A screwdown crown prevents water from entering a watch by sealing it against the case. The seal is achieved by a rubber gasket which presses against the case when the crown is tightly screwed in.
Pushpins
Pushpins or a push-piece is usually mounted next to the crown a control chronograph functions on a watch.