Gallon is an imperial and U.S. customary volume unit. In U.S. customary, there are two types of gallons (dry and liquid) and in imperial system, there is only one type of gallon. It is commonly used in everyday life (except the dry gallons, it's mostly used in measuring the corn, grain and other types of dry goods) from measuring the wine and beer to fuel consumption (measuring the fuel efficiency as miles per gallon). After the metric system is accepted, this volume unit is mostly replaced by liter. The abbreviation is "gal".
Liter is a commonly used metric volume unit which equals to 1 cubic decimeter. The abbreviation is "l".