A mass driver refers to some form of electromagnetic catapault used to launch a payload into orbit faster than escape velocity. A mass driver would use a linear motor, designed to force an object in one direction and then release, as opposed to a rotating motor. Momentum carries the object the rest of the intended course.
Linear motors use electromagnetic force to propel a projectile or transport unit forward. High-velocity linear motors are either railguns or coilguns. Railguns use the Lorentz force, which propels a projectile perpendicular to a flowing magnetic field; whereas coilguns use a sequence of alternatively firing electromagnets to propel a projectile. There is a subtle difference.
All mass driver proposals use coilguns rather than railguns. The sequential and timed nature of the electromagnets allows velocity and acceleration to be more finely determined. This control allows passengers and cargo to be exposed to minimal g-forces.
The mass driver is a technology that intends to upset the chemical rocket. It might end up competing with the space elevator or space fountain. If a lower acceleration factor is desired, a mass driver must consist of a longer, more gradual track. Most track proposals are in the dozens of kilometers (or tens of miles) at least. Some space enthusiasts have suggested the savannas of Africa as the best place to construct a mass driver because of their level ground.
Some mass driver proposals involve a spacecraft propelling bits of material out its rear instead of being fired itself from a mass driver. Combinations of both are certainly possible, and most proposed mass driver designs involve reception drivers designed to extract as much momentum as possible from the incoming projectile.
Mass drivers may one day make cheap interstellar travel possible. Using huge tubes thousands of kilometers long, pods would be launched from star to star at relativistic speeds. Only intervening dust particles would subtract from the object's momentum, while the rest would be recaptured at the other end. Mass drivers could open up the galaxy to travel.