A bubbler is essentially a cross between a pipe and a bong. It's like a pipe because it has the size and shape of a pipe, and also includes a hole or 'carb' on the side of the bowl. But it also usually has a bulb under the bowl to hold water, allowing water filtration just like in a bong. This is useful if you want something portable like a pipe that still has the potency of a bong. It is named for the sound the water makes when you smoke it.