Fuel economy, or fuel efficency, is used to describe how much fuel an engine/car consumes over time. Vehicles with better fuel economy can go further on a given amount of gasoline, or their alternaive fuel of choice.
The distance a vehicle can go on one tank of fuel isn't a good measurement for comparing the fuel consumption of one vehicle to another, because some fuel tanks are larger than others. Instead, a distance per relevant unit of measurement is used, such as miles per gallon or kilometers per liter. A measurement like "MPG", or miles per gallon, is just what it sounds like; the number of miles that can be travelled using a single gallon of fuel. Many modern vehicles have active measurements, and will constantly provide estimates based on the data available, and even the range (since start, since last fill-up, since odometer trip reset, etc.) selected.
If you are interested in improving the gas mileage of your own car, you can read how to do so on our dedicated article, How to Improve Fuel Efficency.
Fun Details
Drag racing vehicles can consume fuel at such a rapid rate, that instead of miles being the defining number, as in miles per gallon, it's gallons per second or gallons per quarter mile.