Definition of Gallons

A gallon is a measure of volume primaririly used in the USA. Most commonly a gallon refers to a US gallon but there are two other gallons out there: imperial gallon - used in the UK and dry gallon mostly used in agriculture (for grapes, apples, berries, etc.)

One US gallon is 231 cubic inches which equals exacly to 3.785411784 liters.

An imperial gallon is approximately 1.2 US gallons or 4.54609 liters. It is based on the volume of ten lbs of water at room temperature.

US gallons are used in the USA and in some Latin American countries. Imperial gallons are no longer the offical measure for volume but many people still use it. In other parts of Eurore liters are used and gallons are not commonly understood. Canada, being a Commonwealth country and bordering the US uses liters officially but most people are good with gallons.

A gallon is divided into four quarts or eight pints.

How to imagine one gallon? What's one gallon in practical life?

Drink 8 pints of beer. That's a gallon. Or 8 large bottles of beer (the one they sell in Europe not the 0.3L ones in North America).
One gallon is also a large jug of orange juice
Or about 11 cans of regular sized pop.