Renewable energy is any sort of energy that can be used now and indefinitely into the future with either little/no environmental impact and without depleting the resource such that it cannot be used in the future. There are many different types of renewable energy and they continue to grow in popularity as cheap effective means of providing energy. They have proven their ability and usefulness and will only become more refined as more people take interest in them.
Perhaps the two most common renewable energies are solar power and hydroelectric plants. Both of these technologies have proven useful in all manner of power generation and storage. Although Hydroelectric power must necessarily be only large scale and somewhat damaging to the environment, solar power can be harnessed at the private level. Anyone with a few extra dollars and the desire can install solar paneling on their very own home to remarkable effect.
Other types of renewable energies include wind turbines geothermal plants and wave and river generators. In some way, all these technologies capture a naturally occurring event and generate electricity from them. River generators use a rivers strong current to collect and store energy while geothermal plants harness the power of geysers and hot springs. We have only scratched the surface of what we can harness from natural processes on our own plane.