A Hall Effect Thruster
is an ion/plasma drive rocket engine for use in space for changing/maintaining orbits. It was invented for Russian 'station keeping', and has been deployed extensively. American versions have also now been tested.
A Hall Effect Thruster consists of a metal cylinder open at one end with a rod up the center. A positively charged ionised gas (a plasma) is introduced near the closed off end, and moves in a generally circular fashion around the rod.
There is a strong radial magnetic field caused by windings in the rod and the outer cylinder. As the charged gas circles the rod, due to the Hall effect, it is strongly pushed along the rod until the spiralling plasma leaves the thruster off into space at considerable speed (around 15km/s).
The vehicle avoids picking up a negative charge by use of an electron gun near the outlet of the thruster. This combines with the exhaust in space and ensures that the exhaust doesn't get reattracted back to the vehicle as that would cancel out the rocket effect.
Hall thrusters are one of the better thruster designs right now. Not only can it give good thrust (by electric thruster standards anyway, 85 millinewtons from a thruster weighing just 3.5 kg), but by ion drive standards it gives a relatively slow exhaust velocity (specific impulse) of very roughly 1500 seconds (see Ion Drive for why this is good.) Partly because of this low velocity, they tend to be of fairly high energy efficiency (2x less energy than ion drives, 2x more fuel than ion drives but 3x less fuel overall than the better chemical rockets.)
Additionally, although a Hall thruster will only work with the fuel it is designed for, it can be designed to run on quite a wide range of common and cheap fuels. For example oxygen can be used (which is much cheaper than say, xenon which other designs utilise). A small percentage of something easily ionised must also be supplied to help create the plasma.