Submersible pumps (or sub pump, electric submersible pump (ESP)) is a device which has a hermetically sealed motor close-coupled to the pump body. The whole assembly is submerged in the fluid to be pumped.
The main advantage of this type of pump is that it prevents pump cavitation, a problem associated with a high elevation difference between pump and the fluid surface. It push fluid to the surface as opposed to jet pumps which create a vacuum and rely upon atmospheric pressure.
Submersibles are more efficient than jet pumps. Hydraulic submersible pumps (HSP's) use pressurised fluid from the surface to drive a hydraulic motor downhole, rather than an electric motor, and are used in heavy oil applications with heated water as the motive fluid.