Fossil fuels are formed from the decayed matter of plants and animals that have been exposed to heat and pressure underneath the earth's crust for millions of years. These once-living organisms are slowly broken down into the hydrocarbons which comprise fossil fuels. The three fossil fuels found on earth are coal, crude oil and natural gas.
Coal is a solid fossil fuel that formed from the decay of vegetation on land. Dead vegetation was compacted and heated, and over millions of years, the carbon and hydrogen in that vegetation became organized into long chain hydrocarbons that release a tremendous amount of heat energy when combusted. Coal is generally found in mines deep below the earth's surface.
Crude oil is a liquid fossil fuel, and it formed from the remains of small marine organisms. It is generally found trapped in small spaces in rocks and is often extracted by drilling into the earth. Natural gas is the gaseous fossil fuel, and like crude oil, it formed from the decay of marine microorganisms. It is composed mostly of methane and is obtained by drilling.
Because there are limited amounts of fossil fuels available on earth, they are considered to be limited resources. They are also considered to be non-renewable resources, since producing more fossil fuels takes hundreds of millions of years and cannot be done artificially.