Gravity Flush - These are the most common; when the toilet is flushed, gravity pulls water from the tank to wash the bowl.
Pressure-Assisted - These use the pressure in your water lines to compress air in a pressure tank, which is released along with the water when the toilet is flushed. This is a more forceful flush often clears the bowl better than some gravity toilets, but can be noisy, and requires a minimum household pressure of 25 psi.
Vacuum-Assisted - These have a chamber in the tank that pulls air out of the trap beneath the bowl to suck water downward with more force.
Dual-Flush - These toilets are most often gravity toilets, and have both a half-flush and a full-flush option. Some have two buttons or a split button on the top, others have a traditional flush lever that can be pushed up or down to vary the flush volume.