Leaded gasoline is gas that has been mixed with a lead-containing chemical, tetraethyllead, whereas unleaded gasoline does not have the added lead. Gas with lead in it was phased out in the United States starting in 1973 and was completely banned for on-road vehicles in 1996.
Lead was added to gas in order to prevent preignition, which is bad for the engine and can result in internal combustion engines continuing to run even after the key has been turned off. Leaded gasoline also coated older engines by covering some of the parts with thin layers of lead. However, in newer cars, the lead in gasoline ruined catalytic converters, which is not a problem with the newer additives in unleaded gas.