Test of a plasma rocket thruster.
The point of a plasma rocket engine is to greatly decrease the time that it takes to get from earth to another planet, like Mars. Using conventional rockets, it can take years to get something to Mars, but using these plasma engines, we might be able to get a craft to Mars in a little over a month. This is amazing because it would keep astronauts from being exposed to too much radiation, as well as solving the logistic problems associated with being in space for several years.
Being in space for years requires bulky craft like this
Plasma rockets are not capable of accelerating nearly as fast as conventional rockets, but they have the advantage that they can accelerate for far longer, and much more efficiently. The general idea is that you use magnetic fields to direct plasma, and thus get thrust. When you are in the vacuum of space, with minimal gravity to slow you down, you can use plasma engines to continually accelerate your craft to extreme speeds, faster than any conventional rocket could ever hope to travel. For this reason, plasma rockets are mainly being looked at as a means of deep space travel.
Rendering of the VASIMR plasma rocket. This rocket has already been successfully tested.
On a side note, I've obviously been having trouble posting. Things are fairly demanding, and I have no idea when my schedule will let up, but as soon as it does, I'll post as often as possible.