Its expansion is temporarily held up by the influence of environmentalist ideology. See the Nuclear FAQ for details. Nuclear energy is humanity's best hope for the long term. These terms are often confused, because in ordinary language they are sometimes synonyms. However, the distinction is important for science and engineering and is simple to state.
Power is the rate at which energy is generated or used.
The basic unit of energy is the joule, and the basic unit of power is the watt. A watt is one joule per second. Thus a hundred watt light bulb is using 100 joules of energy per second, turning the electric energy into light and heat. A 1,000 megawatt power plant is generating a billion joules per second. It gets that energy from burning fuel or from splitting atoms of uranium and plutonium.