In a bong, when you want to pull your hit through, you take out the bong. In a bubbler, you let go of the shottie (a hole in the side).
The concept behind a bubbler is similar to a pipe, except there is water involved.
The result was a wonderful hybrid for smoking cannabis. It resembles a large pipe with a massive bulb under the bowl, which is meant to hold water, with a glass stem dropping from the bowl into the water, which cools the smoke in true bong style.
Yeah, it's a bubbler. You get the best of both worlds (and get marvelously blitzed)!
Out-of-Stater: What the hell is a bubbler?