A renewable energy source is a source of energy that is not likely to run out within our lifetimes. The wind will never go away. Neither will the Sun. Although we know that the Sun is going to explode, since it is a star, it is not likely that it will happen in our lifetimes. Water is also not likely to run out completely within our lifetimes. However, coal on the other hand will run out some day; it takes millions of years to make coal, and people are using it faster than it can replenish itself.
A good way to remember which energy source is renewable and which is not is to look at how much it pollutes. Fossil Fuels such as Coal, Oil, and Natural Gas pollute a lot, and they are all nonrenewable energy sources. On the other hand, clean energy sources such as solar power, hydropower (water power), and wind power do not pollute as much, and they are all renewable resources. (Please note that this relationship is not causation, but rather correlation. The amount of pollution an energy source emits does not cause it's renewability.)
Energy is the ability to do work. The form of energy that living things need for these processes is chemical energy, and it comes from food. Autotrophs make their own food. Heterotrophs obtain food by eating other organisms. Organisms mainly use the molecules glucose and ATP for energy.
