Uranium is the element having 92 protons in its nucleus with typically 146 neutrons. It is the largest naturally occurring element. Like naturally occurring Thorium, uranium is radioactive and eventually decays into radium and radon which are likewise radioactive. Both uranium and thorium decay through many series of radioactive elements until they eventually become lead.
The process of radioactive decay follows a common yet generally unfamiliar law known as exponential decay. It is from this decay law that we get the term, "half-life". The way this works is that when you start with a fixed amount of a radioactive material, if you wait through its half life, half of the material will have undergone radioactive decay. That remaining material which had not undergone radioactive decay will again have half of that amount undergo decay in the amount of time representing the next following half life. So with each half life, half of what is left will decay.
In this sense, if a radioactive isotope has a half life of 1 year, the amount left at the end of the first year is 1/2, the amount after the second year is 1/4 the original and the amount left after 3 years is only 1/8 and so on. After 10 years, the remaining amount of radioactive material which has not decayed would then be less than 1 tenth of 1 percent of the original (or more exactly, one half to the tenth power).
This system can get much more complicated when a radioactive isotope decays into another different radioactive isotope with a different half life. In the case of uranium and thorium, these have up to 12 sequential radioactive decay products in a series (uranium has 12 and thorium has 9). These decay products include different isotopes of radium and radon.
An isotope is an atom with the same number of protons which define it as an element but different amounts of neutrons. So although all uranium atoms have 92 protons, some have only 143 neutrons. Adding up the numbers of neutrons and protons in this case give 235 total and so this isotope of uranium is known as U235. The more common form of uranium described earlier has a sum of 238 neutrons and protons and so is known as U238.
The U235 isotope is only found at very low concentrations in nature, less than 1%. Furthermore, U235 is the only isotope which is also fissile. To be fissile, an isotope must be able to absorb a slow neutron and then split into multiple nuclides giving off one or more neutrons in the process. These free neutrons are then able to be absorbed by other fissile nuclides (i.e., U235) carrying on the process.
This means that U235 can give off more than enough neutrons such that under carefully controlled conditions, it can sustain a chain reaction. In chemistry, a chain reaction would simply be an ongoing fire or a rusting metal bar in water. Basically a chain reaction is one that is able to maintain itself under specific conditions such as sufficient fuel. A nuclear chain reaction on the other hand is much more difficult to create and sustain and requires higher than current natural U235 concentrations.
So in order for nuclear fuel to support commercial electricity generation, the naturally low enrichment of U235 has to be increased to allow a nuclear chain reaction to be engineered. The heat from the nuclear chain reaction is able to boil water to create steam in the same way that a natural gas or coal fired power plant creates steam to run through a turbine and create electricity.
The half life of U235 is much smaller than that of U238 so that far enough back in time, natural uranium would have been enriched more than current commercial nuclear fuel. Basically the U235 decays much faster than U238 and so today, there is much less U235 than there was in the distant past. Modern scientific methods have found evidence that a natural nuclear reactor once did occur on earth (when there was much more U235 around) and actually went critical similar to that in modern nuclear power plants.
The only known natural nuclear reactor in earth's history was found in the Oklo uranium mine of Gabon, Liberia on the continent of Africa. Some of the U235 was missing and the stable fission decay products present in the mine gave evidence that a natural fission reaction process occurred a very long time ago.
Many have argued that this is evidence that radioactive waste can be safely disposed of in deep geologic formations. The idea being that as Oklo kept the fission products long enough to fully decay into stable elements, duplication of this design would then be technically feasible.