Fuels such as petroleum (and thus gasoline or diesel), coal, and natural gas, which are derived from remains of prehistoric vegetation and animals are called fossil fuels.
Source(s):
E.g. http://www.merriam-webster.com/dictionary/fossil%20fuel

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Fossil fuels or mineral fuels are fuels formed by natural resources such as anaerobic decomposition of buried dead organisms.

-quote-

"Fossil fuels or mineral fuels are fuels formed by natural resources such as anaerobic decomposition of buried dead organisms. The age of the organisms and their resulting fossil fuels is typically millions of years, and sometimes exceeds 650 million years. These fuels contain high percentage of carbon and hydrocarbons.

Fossil fuels range from volatile materials with low carbon:hydrogen ratios like methane, to liquid petroleum to nonvolatile materials composed of almost pure carbon, like anthracite coal. Methane can be found in hydrocarbon fields, alone, associated with oil, or in the form of methane clathrates. It is generally accepted that they formed from the fossilized remains of dead plants and animals by exposure to heat and pressure in the Earth's crust over hundreds of millions of years. This biogenic theory was first introduced by Georg Agricola in 1556 and later by Mikhail Lomonosov in the 18th century."

-end of quote-
Source(s):
http://en.wikipedia.org/wiki/Fossil_fuel

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Fossil fuels are the nation’s principal source of electricity. Fossil fuels come in three major forms: coal, oil, and natural gas. Because fossil fuels are a finite resource and cannot be replenished once they are extracted and burned, they are not considered renewable.
Source(s):
www.epa.gov/greenpower/pubs/glossary.htm

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If a living organism dies and gets covered up by mud or dust quickly enough for oxygen to not be able to decompose it completely into water, nitrates, and carbon dioxide, then the organism will only partially decompose into hydrocarbons (oil) and methane (natural gas).

In both cases, what they have in common is that there was no oxygen for the organism to decompose all the way.

For billions of years organisms have been dieing in water, or dieing close to water and falling it, whereupon sometimes it sinks to the bottom and gets covered with mud fast enough that oxygen doesn't have a chance to get at it.

The mud then gets covered by more mud, and eventually gets pressed into shales, and then as the tectonic plates of the earth's crust push and grind against each other that shale gets rolled up and folded and kneaded like puff pastry with lots of shortening, such that like puff-pastry which has little pockets of shorting all through it, so also the earth's crust has little pockets of unoxidized hydrocarbons and methane all through it.

Because those hydrocarbons and methane were incompletely oxidized, if you can find some, and trigger it with a bit of heat, like from a spark, then, if there's oxygen, it will jump an activation energy hurdle and quickly complete the oxidization process all the way to water, nitrates, and carbon dioxide, releasing lots of stored chemical bond energy in the process... called "burning".

Humans were already accustomed to burning wood, and when they discovered that they could also burn those incompletely oxidized hydrocarbons and methane found in pockets in the earth's crust, they called the hydrocarbons coal and oil, and they called the methane natural-gas.

Because those hydrocarbons and natural gas came from organisms that had been alive but were then quickly covered in mud before oxygen could oxidize their bodies completely, sometimes, if those organisms also had a shell, that shell would leave an imprint in the mud, and we call those imprints fossils.

Animals bodies are what became oil, and so we often find animal fossils around oil, and plants became coal, so we often find leaf-fossils in coal. Both made natural-gas from the un-oxidized nitrogen in their proteins.

Because wherever people find pockets of coal, oil and natural-gas they also often find fossils, they have nicknamed coal, oil, and natural gas "fossil fuels".
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Most of the replies to this question have given a basic "objective science" reply to your question, but I wonder if you are asking something more along the lines of "what are fossil fuels, and why are people making such a fuss about them now?"

This is an important question, particularly because many experts are arguing that it is OK to use biomass as a fuel, but not fossil fuels. But if fossil fuels come mainly from plant biomass in the first place, what is the difference?

The difference has a lot to do with time scale. The atmosphere contains a certain amount of CO2, and global warming is happening because that amount is increasing. Why is it increasing? The experts say because we are burning too much fossil fuels. So why is it ok to use biomass for fuel? The basic idea with biomass fuel is that it comes from plants that were grown specifically to produce the fuel. So although biomass fuels release the same CO2 that fossil fuels do when they are combusted, all of that CO2 was taken up CO2 by the plants when they were grown to produce the biomass. We can think of this as "paying in advance" for the CO2 that is emitted. Fossil fuels also came originally from plants, so the CO2 emitted from burning fossil fuels also is CO2 that was originally taken up from the atmosphere by plants. But this was done over a long long period of time - millions of years. And now we are pumping all of that CO2 back into the atmosphere in less than 100 years. That is why we are seeing the sharp increase of CO2 concentration that scientists have been tracking and people like Al Gore have been bringing to the popular media.

I hope that you find this answer useful.
Source(s):
I used to teach and do research related to this stuff, so my source is just what I was able to pull out of my head. :-)

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Fossil Fuels are energy rich substances formed from long buried plants ,ancient organisms and microorganisms. Fossil fuels also called as 'Mineral fuels' are formed as a result of natural resources such as Anaerobic decomposition of the buried micro / macro-organisms and take centuries to form.
Fossil fuels principally consist of carbon and hydrogen bonds, in short, the remains gradually get transformed into Hydrocarbons.
The fossil fuels are formed as a result of accumulating sediment layers. As the sediment layers go on adding over years, there is an increase in temperature as well as pressure and after hundreds of years due to these physical conditions the organic materials get transformed into
hydrocarbons.
The most commonly used fossil fuels are petroleum, coal, diesel and natural gas. These substances are extracted from the earth’s crust and, if necessary, refined into suitable fuel products.
The reason these fossil fuels are used is that they produce significant amount of energy. The most commonly used fossil fuels are natural gas, coal, petroleum..which we use on a larger extent.
Fossil fuels are non-renewable resources i.e., these are natural resources which cannot be regenerated and are exhausted when once used. They take millions of years to form, and reserves get depleted much faster until the ones are formed.
Also, since that fossil fuels are non renewable resources, many alternatives are now been used like hydroelectric energy, geothermal energy, solar energy, wind energy, etc. which can be made available in abundance and the fossil fuels are also used in a limit.
Hope this helps a bit.
Thanks.
Source(s):
http://encarta.msn.com/encyclopedia_761586407/Fossil_Fuels
http://en.wikipedia.org/wiki/Fossil_fuel
http://www.lenntech.com/greenhouse-effect/fossil-fuels.htm

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Fossil fuels are hydrocarbons, primarily coal, petroleum, fuel oil and gas. They formed from the fossilised remains of dead plants and animals by exposure to heat and pressure in the Earth's crust over hundreds of millions of years.
Source(s):
http://www.examville.com

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