Tube-style
This form of bong looks like a tall tube / cylinder with straight glass. One end is closed, and the bowl sticks off the side of the pipeline.

Beaker style

This form looks much like the beakers used by chemists, but in fact, because the bottom is flared, they are just like tube bongs. Otherwise they essentially work the same as a tunnel. A major advantage over tube-style bongs, when put on a flat surface, is that they are more stable. Go for a beaker bong, if you fancy yourself a klutz.

Round-base

Another form consists of a tube rounding out at the end, essentially into a sphere with the exception of its flat bottom so that it can sit on top of a surface. Otherwise it just acts like a beaker bong or tube bong.

Percolator or "bubbler"

That type may come in various types. What describes it as a "percolator" rather than a bubbler is that it has inside a component that dissipates the smoke and then sends it through the chamber water. This allows the water to "bubble" or "percolate." This process added provides additional filtration and also has a cooling effect on the smoke.

Gravity style

This form of bong, as the name implies, harnesses the power of gravity, or more precisely the vacuum that is generated when water leaves an enclosed space. The vacuum from the pipe draws in smoke that is then inhaled. A gravity bong may be in the form of "bucket" or "waterfall," but the fundamental science behind the process is similar.

For a bong without a percolator, before the smoker inhales it, the smoke gets filtered through the water in the water chamber. This process removes most waste, and cools down the smoke. For a unit with percolator, after making its way through the water chamber, the smoke passes through the percolator. The percolator removes even more residue and cools the smoke further, which results in a hit that is cool rather than warm or hot and feels much smoother.