Oil is the product of the decomposition of the remains of microscopic living organisms which millions of years ago inhabited the seas, lakes and river estuaries. It is a liquid substance, which is less dense than water, has a dark and oily appearance and a strong smell, and is formed by a mixture of hydrocarbons (chemical compounds which only contain carbon and hydrogen in their molecules).

Oil today has a multitude of applications including: petrols, diesel, fertilizers, plastics, explosives, medicines, colourants, synthetic fibres, etc. This is why is it essential not to waste it simply as fuel.
The following visual shows how it is formed and how it is used.