Fuels are substances or energy sources that are burned or consumed to produce heat, power, or mechanical energy. They are typically used in transportation, electricity generation, heating, and industrial processes. Fuels can be categorized into different types based on their source and composition: fossil fuels (coal, oil, natural gas), renewable fuels (biofuels, solar, wind, hydro), and nuclear fuels. The combustion of fuels releases greenhouse gases and other emissions, contributing to climate change and air pollution.