2.1 Fossil fuels

Primary energy sources take many forms, including nuclear energy, fossil energy -- like oil, coal and natural gas -- and renewable sources like wind, solar, geothermal and hydro power. These primary sources are converted to electricity, a secondary energy source, which flows through power lines and other transmission infrastructure to your home and business.

(Source : http://energy.gov/science-innovation/energy-sources (Links to an external site.)) 

Fossil fuel is a term used to describe a group of energy sources that were formed when ancient plants and organisms were subject to intense heat and pressure over millions of years.

There are three major types of fossil fuels:

• Coal is formed from ferns, plants and trees which hardened due to pressure and heat
• Oil is formed from smaller organisms, like zooplankton and algae. Intense amounts of pressure caused this complex organic matter to decompose into oil. 
• Natural Gas undergoes the same process as oil; however the process is longer and subject to higher amounts of heat and pressure, causing further decomposition.
  

 (Source : https://www.studentenergy.org/topics/fossil-fuels#reference-3 (Links to an external site.))