Enriched uranium is uranium with a high percentage of the isotope U-235, which only makes up about .72% of natural uranium. Normal uranium is referred to as U-238, where the number signifies the amount of nucleons (protons and neutrons) in its atomic nucleus. U-235 has an uneven quantity of protons and neutrons, making it slightly unstable and susceptible to fissioning (splitting) from thermal neutrons. Getting the fission process to proceed as a chain reaction is the basis of nuclear energy and nuclear weapons.
Because U-235 has identical chemical properties to normal uranium and is only 1.26% lighter, separating the two can be quite a challenge. The processes are usually quite energy-intensive and costly, which is why only a few countries have been able to achieve it an industrial scale thus far. To make reactor-grade uranium, U-235 percentages of 3-4% are required, whereas weapons-grade uranium must consist of 90% U-235 or more. There are at least nine techniques for uranium separation, although some definitely work better than others.
During WWII, the United States, when researchers were first pursuing isotope separation, a series of techniques were used. The first stage consisted of thermal diffusion. By introducing a thin temperature gradient, scientists could coax lighter U-235 particles towards a region of heat, and heavier U-238 molecules towards a colder region. This was just preparation of feed material for the next stage, electromagnetic isotope separation.
Electromagnetic isotope separation involves vaporizing uranium and then ionizing it to produce ions with positive charge. The ionized uranium was then accelerated at bent by a strong magnetic field. Lighter U-235 atoms were deflected slightly more, while U-238 atoms slightly less. By repeating this process many times, uranium could be enriched. This technique was used to make some of the enriched uranium for the Little Boy bomb, which destroyed Hiroshima.
During the Cold War, electromagnetic isotope separation was abandoned in favor of the gaseous diffusion enrichment technique. This approach pushed uranium hexafluoride gas through a semi-permeable membrane, which slightly separated the two isotopes from one another. Like the prior technique, this process would have needed to be performed many times to isolate a substantial amount of U-235.
Modern-day enrichment techniques use centrifuges. The lighter U-235 atoms slightly preferentially pushed towards the outer walls of the centrifuges, concentrating them where they can be extracted. Like all the other techniques, it must be performed many times to work. Full systems that purify uranium in this way utilize many centrifuges and are called centrifuge cascades. The Zippe centrifuge is a more advanced variant on the traditional centrifuge which utilizes heat as well as centrifugal force to separate the isotope.
Other techniques of uranium separation include aerodynamic processes, various methods of laser separation, plasma separation, and a chemical technique, which takes advantage of a very slight difference in the two isotopes' propensity to change valency in oxidation/reduction reactions.