Water screens are usually used indoor for generating a projection surface that can be easily and quickly switched on and off. Water screens provide a curtain of a multitude of water drops falling downwards. The sreecn consists of the actual water screen element with rows of nozzles that are in very close distance to each other, a strong pump with a special hose and a water basin. To have more depth in projection surface and a better "resolution", water screens usually have two or three nozzle rows. The more nozzle rows, the more water density in the actual projection surface.
Water screens are ususally used to project on with video and/or lasers. There are also solutions that use the water screen as effects element itself, however this is a very rare kind of application.
More advanced water screen solutions are graphics capable themselves, meaning the water drippling from the single nozzles can be controlled to generate text or patterns of water falling from top to bottom. Together with lasers this creates amazing effects!
Waterscreens consist of a nozzle element that is equipped with hundreds of nozzles in several rows. This element is mounted to the ceiling, truss, etc.. A pump delivers water from a water source to this element and makes water dripple out of the many holes. So the general principle of a water screen works opposite to the water shield: Whereas the water shield emerges from bottom upwards, the waterscreen is created of water drops drippling down - that's why some people also call it "water curtain". Water screens are normally used at indoor applications, hydro shields more for larger scale outdoor projections. With lasers it is possible to project on the water screen from both directions, video projections should face the spectators to be properly visible.