Vaporizers heat the marijuana to the point where the THC vaporizes, but doesn't get hot enough to actually burn the plant. It eliminates a lot of the extra shit you inhale when you actually smoke weed, and is much much more healthier for your lungs.
I don't own one myself, but I've only heard good things about Volcano vaporizers. I'm sure someone else will chime in with better advice on which one to get, though.