The litre (or liter; US spelling) is a unit of volume and is a non-SI metric unit with the symbol L).
1 litre is equal to the volume in a cube with edges all measuring 10cm. There are 0.22 imperial gallons in a litre. Conversely, 1 imperial gallon is equal to 4.5461 gallons.
1 litre of water weights exactly 1 kilogram.
After the metric system was introduced in France in 1791, it took a couple of years for the entire country to implement it in everyday use. After much backlash, it was decided that the cubic metre was too big for everyday use. By 1795 it was announced that the former 'cadil' (0.001 cubic metres) had been given a new name; 'litre'.
The cup is a unit of volume in the US customary unit system with the symbol cup.
It is equal to 1/2 US customary pint, 1/4 US customary quart and 1/16 US customary gallon. it is also equal to 236.5882365 millilitres.
Used primarily for cooking - the cup was adopted and established as a recognised unit of measure as it could be used by almost anyone in any kitchen. Due to the slight variants in design and size, it is rarely used as an accurate measure for important or critical materials, however it does represent a specific amount and can be used across many recipes to indicate what is needed.