Fossil fuels are non-renewable energy resources. Their supply is limited and they will eventually run out. Fossil fuels do not renew themselves, while fuels such as wood can be renewed endlessly.
Fossil fuels release carbon dioxide when they burn, which adds to the greenhouse effect and increases global warming. Of the three fossil fuels, for a given amount of energy released, coal produces the most carbon dioxide and natural gas produces the least.
Coal and oil release sulfur dioxide gas when they burn, which causes breathing problems for living creatures and contributes to acid rain.
Check Fuels from crude oil to make sure you understand these points.