Energy exists in various forms, including mechanical, thermal, chemical, electrical, gravitational, and nuclear and are all interconvertible. Mechanical energy results from movement and is the combination of kinetic and potential energy. Thermal energy is the outcome of temperature differences between two systems. Electromagnetic energy (also called radiant energy) is the outcome of electromagnetic waves such as light emitted by the sun. Gravitational energy is the foundation of mechanical energy derived from the attraction of two masses, the earth being the most significant.
Forms of energy come from sources qualified as renewable and non-renewable, which include chemical reactions (mainly combustion), nuclear reactions (fission or fusion), the effect of gravity (mainly tidal) and direct (photovoltaic) and indirect (photosynthesis, wind and hydraulic) solar energy conversion.
The concept of renewability is based upon the scale of human events and if the source can be replaced during that period. Fossil fuels are the most common source of non-renewable energy since oil or natural gas reserves would take millions of years to replenish themselves through anaerobic decomposition. Inversely, wood is a renewable biomass energy source as long as adequate conditions are kept for reserves to be replenished. Rates of exploitation / deforestation in a number of areas are so high that biomass may be considered as a non-renewable source in those circumstances.