Green Shells are the basic ranged offensive item. You can throw them either ahead of you or behind, and it will travel in a straight line until it comes into contact with a wall or, even better, an intended opponent. Green Shells are generally hard to hit people with, so a few skills in timing and just your position in correlation with the other guy can help. If you get hit by one, you flip over, similar in damage to other hindrances. A Green Shell can bounce off a barrier up to five times. The final ricochet will destroy it.
In addition, Green Shells can be carried behind you as a shield from other Items, such as the more devastating Red Shell.