It's a pretty self-explanatory term: 'gallons per flush' (GPF) refers to the amount of water a toilet uses, to um, do its job. However, not all toilets are created equal, and it's important to understand how many GPF are going down your drain.
Lift up the cover on your toilet tank and take a look at the underside. It may indicate straight up how many gallons per flush the particular model uses, but it might only provide the year in which it was manufactured. This bit of information still tells you a lot. If your toilet was manufactured before 1992, it likely uses several gallons of water for each flush.
Things changed for toilets after this date; the Energy Policy Act required that toilets use no more than 3.5 GPF. These days, most low flush toilets use less than 1.6 GPF. The Environmental Protection Agency's WaterSense program labels such toilets and claims that installing one "can save a family of four, on average, $2,000 in water bills over the lifetime of the toilet." Another way to drastically reduce your GPF is to install a composting toilet system, which uses very little water.