The barrel is a unit of volume used in the oil industry with the symbol bbl.
Oil is more commonly expressed in terms of cubic metres (m³) but in the US and in the economic world (mainly because such a large percentage of the world's oil is shipped to the US) as barrels.
The measurement of the barrel originated in the US at the first oil well in Pennsylvania. There were so many variants of barrel size (including those used for wine and other goods) that the distrust became so prevalent a central and standard barrel size was created. This was finally established in 1872 as 42 US gallons.
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'.