In a steam engine, a steam locomotive for example, or a stationary power plant, saturated (or "wet") steam is steam in the condition in which it is tapped directly off the top of the boiler. It is just a little above the boiling temperature of water at the boiler pressure.
While an engine using saturated steam works quite well, it was discovered that a great improvement in efficiency occurs if the steam can be heated to still higher temperatures. This heated steam is known as superheated steam, and the apparatus used to heat it is known as a superheater.