“”"Oil? An emergency? Ha! It's about time the people who run this planet of yours realised that to be dependent on a mineral slime just doesn't make sense."
|—The Fourth Doctor, Terror of the Zygons|
Fossil fuels consist of biological material that had decomposed and been placed under extreme pressure for extremely long periods of time (millions of years). They include coal, natural gas, and petroleum. Fossil fuels are of great importance to the world's energy production as they account for 86% of it. There is great concern over what happens when we run out (peak oil). Use of fossils fuels also has damaging environmental effects (pollution and greenhouse gases).
 How does it work?
When plants or animals die, some of the potential chemical energy in them remains. This matter may sink to the bottom of the ocean or lakes and be covered with sediment. Over millions of years it is compressed by layers of earth, and the same amount of energy will occupy the now smaller space. This in turn makes the matter give off far larger energy output than the more recently deceased. However, as we still are burning almost pure carbon, the main byproduct is carbon dioxide, a greenhouse gas.
 Problems with fossil fuels
As mentioned before, the two main problems with using fossils fuels are the byproducts and the fact that, for all practical purposes, it's non-renewable. The most common method of using coal to produce electricity releases large amounts of the greenhouse gases produced straight into to the atmosphere. While cheap, it certainly doesn't contribute positively to global warming. There are other coal related methods that are less harmful, mainly clean coal burning(citation needed), but due to the ever increasing depletion of coal, it seems an unlikely long term solution.
Due to the process in which fossil fuels form they have a limited supply before the "stockpile" must be left to replenish. This would mean essentially a permanent switch from fossil fuels to another renewable energy source. Currently there are no viable alternatives that can be implemented to all areas where fossil fuels are used.