Its meant to mimic the effects of cannabis but from what I understand it doesn't really do that...causes a different sort of high, can be made from various chemicals. Basically its random plant matter you can smoke with chemicals sprayed on it. Its a lot more dangerous than cannabis but it gets marketed as a 'legal' alternative since its not a scheduled drug so people sometimes resort to it. It also has much more addiction potential than marijuana.

Some call it 'synthetic marijuana' but that is false since Spice does not contain any cannabanoids synthetic or otherwise. Most of all though its just nasty stuff best to stay away from.