Renewable energy is generated from renewable energy sources, sources which can be renewed and hence are practically unlimited in quantity. Some examples are solar energy, wind energy, etc. Fossil fuels, on the other hand, take millions of years to form and hence are not easily renewable (at least not in our life times) and hence energy thus produced is called non-renewable energy. Some examples of fossil fuels are coal, natural gas, petroleum, etc. Most of our current energy requirements are met by fossil fuels and the existing (or known) stocks are dwindling to dangerously low levels. We are probably going to run out of most of these in the next century or so (at our current rate of consumption). And hence there is a major push for using renewable energy options. Many of these options are also very clean, i.e. they do not produce emissions such as Carbon dioxide, etc. Fossil fuels, on the other hand, generate these gases upon combustion and cause the greenhouse effect.