Petroleum (crude oil) is a thick, flammable, black-to-yellow mixture of solid, liquid and gaseous hydrocarbons believed to be the remains biomass matter accumulated over millions, if not billions, of years below the Earth’s surface.  One barrel of crude oil contains 42 gallons.  The typical barrel makes:

Product Gallons
Gasoline 19.4
Distillate Fuel Oil (home heating oil and diesel fuel) 9.7
Kerosene – Jet fuel 4.3
Coke 2

Residual/Heavy Fuel Oil (Fuel Oil No. 6 or Bunker C) is what's left at the end of the refinery process.  These heavy oils are used as fuel in ships, industry and for electric power generation.

1.9
Liquefied Refinery Gases 1.9
Still Gas 1.8
Asphalt and Road Oil 1.4
Petrochemical Feedstocks 1.1
Lubricants 0.5
Kerosene 0.2
Other 0.4
Total 44.6

The total volume of products made is 2.6 gallons greater than the original 42 gallons of crude oil.  This represents “processing gain.”  Refinery processing gain represents the volumetric amount by which total output is greater than input for a given period of time.