When we talk about renewable energy, we refer to energy sources that will always be there, at least as far as we're concerned. The sun, theoretically finite, will shine for an estimated five billion years, making its energy infinite from a practical standpoint. As long as there is sun, we will have wind and and other energy forces from the temperature gradients of the earth. We will also have biomass sources, in the form of trees, plants, and creatures. These will be available to us as long as our use does not outpace their rate of regeneration. All forms of energy are really solar energy, except for nuclear energy from uranium or geothermal energy, which comes from the earth's core.
Renewable energy comes in many forms. The wind blows because of temperature differences among masses of air and varying topography, primarily created by the sun's heat. Wood and other biomass energy crops like corn and sugar cane collect the sun's energy as they grow. Fossil fuels are merely forms of ancient biomass energy compressed over millions of years into purer, more highly concentrated forms of carbon. The rivers that power hydroelectric dams flow because of the continuous watershed cycle of evaporation and rain or snow, a process in which the sun's heat plays a major role. And, of course we feed our bodies energy in the form of plants and animal proteins that rely on the sun for their existence.
What will we use after fossil fuels? Logic says it will have to be something we will continue to have enough of, and it will have to be environmentally friendly. In other words, it will need to be renewable energy. We have plenty of sun, wind, and organic matter. There is lots of hydrogen, as well as geothermal heat from inside the earth. There is plenty of wave power and river power. There is no question that theoretically they can more than do the job (Kelly, 2008, p.132-133).
Kelly, I.(2008). Renewable Energy. In Energy in America: A tour of our fossil fuel culture and beyond. Burlington, VT: University of Vermont Press.
http://needtoknow.nas.edu/energy/glossary/ <---- energy glossary