Plutonium has 94 protons in its nucleus, compared to the 92 in uranium, and is typically produced in nuclear reactors as a byproduct. Although both plutonium and uranium are rare, naturally occurring elements, plutonium is no longer found in the earth's crust, but uranium is actively mined. There is enough uranium inside the Earth that its radioactive decay is believed to heat the Earth's core.
The two most common isotopes of uranium are U-235 and U-238, but U-238 is much more common than U-235. Of the two, U-235 is fissile and is readily burned in reactors. U-238, however, is capable of capturing a neutron in the reactor and becoming Pu-239, an isotope of plutonium.
Plutonium is capable of producing more energy than uranium. One kilogram of Pu-239 is sufficient to produce nearly 10 million kilowatt hours of energy, whereas 1 kilogram of uranium produces about 500,000 MJ of energy, or about 140,000 kilowatt hours. However, most reactors end up burning both uranium and plutonium.
The half-life of U-238 is very long, roughly 4.5 billion years, and the half-life for U-235 is only a little shorter. In contrast, the half-lives of various isotopes of plutonium range from 14.4 years in the case of Pu-241 to 82 million years in the case of Pu-244. In any case, uranium has a longer half-life than plutonium. This longer half-life is why uranium is still found in the earth's crust, whereas the plutonium has already decayed.