So, what is the difference between a plasma cannon and a plasma rocket? Well, a plasma cannon does not generate thrust. The exiting plasma stream is made turbulent, and in a process akin to cavitation, that turbulence negates the potential thrust. In a plasma rocket the exiting stream is magnetically coupled to the chassis, the turbulence is minimized, and you get some "holy crap" kinds of thrust.

Sounds like science fiction, right? Well, it's science fact, folks. Have a look at this article in Chalain's Science Corner, if you haven't already. It's fun, and it's funny. Oh, and most of the content actually came from NASA.