BongA water pipe or bong is a device used for smoking cannabis, in which smoke is bubbled through a chamber containing water. Water pipes derive their heritage from the classical Arabic hookah, which was originally designed for smoking tobacco and other flavored smoke producing materials. Bongs differ from hookahs in that they are usually simpler, with a single mouthpiece on the water chamber rather than a hookah's one or more hoses.
Bubbling the smoke through water serves to cool down the smoke and to trap some of the heavier and more soluble particulate matter, keeping it from entering the smoker's lungs. For this reason, some users claim that cannabis smoked through a water pipe is safer than many other recreational drugs.
A gravity bong is a different smoking device which also uses water. They are almost all home-made and are rarely available commercially. A gravity bong is made of two different containers, most typically a bucket and two-liter soda bottle. The bottom of the bottle is cut off, and the cap is made into a device capable of holding the substance to be smoked. The bucket is filled with water, and the bottle placed in the water so the missing side is underwater and the cap, filled with the to-be-smoked substance, is above it.
The substance is lit by one person while another slowly and steadily raises the bottle, careful not to bring the edge of the open bottom above the water level. The increasing volume of waterless-space inside the bottle pulls smoke from the cap into the bottle. The cap is taken off, and one person inhales the smoke inside the bottle, often while plunging the bottle back into the bucket (in order to force more smoke into the lungs).
A gravity bong, unlike a bong proper, does not filter the smoke or cool it significantly. It is simply a device for concentrating smoke and making it easier to inhale a large quantity quickly. Indeed, the term is even more of a misnomer, as the device does not rely on gravity whatsoever but rather on air and water pressure.