A jet pump is a fuel or air pump without moving parts. In the case or a jet fuel pump, a small amount of fuel from the aircraft engine driven fuel pump is routed to the jet pump. It is squired through a passage that is a venturi. As it passes through the venturi, pressure drops. This pressure drop is used to draw fuel out of the tank, and is the "pumping action." The jet pump uses existing fuel flow to move other fuel. The only control needed is a valve somewhere in the system to shut off or allow to flow the existing fuel, which is also called the "motive fuel."
The same thing occurs in most pressurization systems. Generally somewhere in the system is a jet pump. It pushes air, usually bleed air, through a venturi, causing a pressure drop. Exactly like what happens in an older venturi tube on the side of the an older airplane, or what happens in your carburetor. This lower pressure creates reduced air pressure in a line, and is used as a reference source or vacum source to get the pressurization controller and components do do certain things (such as open outflow valves, etc).
A jet pump is nothing more than a valve that uses an existing fluid, fuel or air, to move the same kind of fluid around. It does so by squiring fluid through a restriction to create a pressure drop.