1) Renewable energy sources can be replaced naturally, at a pace fast enough to replenish current use. This allows renewable energy sources to be theoretically used over an infinite length of time. On the other hand, nonrenewable energy either cannot be replaced naturally, or does so at a very slow pace which would not allow it to be used perpetually.
2) One example of renewable energy is wind energy. Because wind is generated naturally by the atmosphere and air currents, no further input is needed on the human side, and winds are not expected to disappear anytime soon. Another example is hydroelectric energy. Due to the water cycle, waterfalls and other hydroelectric dams are unlikely to "run out of water" anytime, and they generally keep up a consistent stream of energy generation.
