Fossil fuels are the result of the fossilization process. In simpler terms, plants and animals that were buried deep down in the Earth's subsurface converted over to petroleum, natural gas, etc. due to pressure and temperature, over a period of millions of years. Fossil fuels include coal, petroleum and natural gas. These have high calorific value, that is they generate lot of energy when burned. Fossil fuels form the backbone of industrial development around the world and are the chief source of electricity (and heat) for almost all countries. The problem with the use of fossil fuels is the carbon dioxide, among other gases, that they release. Some of these gases have been found to contribute to global warming. Another issue is their limited availability (due to the time required for their formation and our ever-increasing energy requirements). Because of these reasons, more and more governments are pushing for adoption of renewable energy options.