A source of energy would be considered renewable if it is a natural resource on earth and it can be naturally replenished on a relatively short time scale. Furthermore, producing electricity from renewable sources will not produce harmful pollutants or emissions and will not harm ecosystems. Renewable Energy is also called "clean" or "green" power.
Fossil fuels like coal, oil, and natural gas are nonrenewable, because they are finite resources which will run out. Fossil fuels are also environmentally harmful to extract and convert into usable energy.
They produce a lot of hazardous emissions and pollutants that contribute to climate change and can harm plants, animals, and humans. Nuclear power is also considered nonrenewable because of the toxic waste it creates.
Renewable energy can be produced using sources like the wind, sunlight, hydrogen, geothermal energy (heat from inside the earth), biomass (energy from plants), flowing rivers, and even the power of the ocean!
Believe it or not, the source of most forms of renewable energy can be traced back to the sun! Solar energy is the most direct use of the sun: sunlight is used to excite electrons and push them through a circuit. Wind energy relies on the sun too! Wind is created by uneven heating of the earth's surface. When sun hits one part of the earth more directly, it warms that part up. The warm air rises and cooler air rushes in, creating wind. So wind energy is actually a form of solar energy too! Of course, energy from biomass relies on the sun too, because plants cannot grow without sunlight! Even hydropower would never work without the power from the sun!
WHY DON'T WE USE MORE RENEWABLE ENERGY?
The use of renewable energy is increasing at very fast pace, but we cannot rely exclusively on renewable energy to meet all of our demand for electricity. Renewable energy is not easy to store or save, because we can only get wind energy when the wind is blowing or solar energy when the sun is shining. Coal and natural gas power plants can store the fuel and use more when there is more demand or less when there is less demand.
Renewable energy is also usually more expensive than conventional energy sources. However, as technology for renewables improves and the cost of fossil fuels increases, renewables will become competitive or cheaper than fossil fuels.