Most modern toilets use about 1.6 gallons of water per flush (as opposed to as much as 5 gallons of water per flush used by old toilets). A dual-flush, ultra low flow toilet might use only 1/2 gallon per light flush and up to 1.6 gallons only for a “heavier”, or more thorough flush.
Toilets operate in one of two ways. Gravity-flush models, which are the most common in residential homes, flush by using gravity to pull down water stored in the tank. Pressure-assist models, which often have no tank, force compressed air and a small amount of water into the bowl and clean it by using greater force. Pressure-assisted styles are more effective and use less water, but they are considerably more expensive to purchase and to maintain.
The best way to shop for the design you like that uses the least water is to compare the amount of water used per flush. This information should be readily available in a home improvement store, bathroom fixtures store or catalog.