A steamroller is basically a tube with a bowl at one end, like a normal piece. However, instead of having a small hole for the carb that you'd cover with your thumb or your finger, its got an open end, that you cover with your whole hand. It makes clearing the smoke out of the chamber easier and quicker. Which means big, hard hits. I love steamrollers.
If you're interested in ordering off of Grasscity and getting a steamroller, I think this is the only one they have. Its kind of pricey, but damn its sick.
Good luck.
http://shop.grasscity.com/shop/grasscity/mjgcmks2.html