a
power source is a
device that supplies some form of
energy, though it's most used to describe sources of
electric
power, like an electrical
generator or a
battery.
Most power sources work by
converting one type of energy,
like
chemical,
nuclear, heat or motion energy, in to
the form of energy desired. The most important energy source
we have is the sun, since only nuclear energy is totaly
independant(Actually the sun is a nuclear reactor, since
it gets it's energy from a
hydrogen fusion process, so you
could say all our energy come from nuclear reactions.)