Natural Uranium contains a number of isotopes in different concentrations. The two primary isotopes of interest are U235 and U238 which have natural abundances of .72% and 99.275% by mass, respectively. For most reactors and nuclear applications, U235 is the primary fissile nuclide. Generally, the percentage of this isotope needs to be increased. This increase in the percentage of U235 is called enrichment. Typical methods for enrichment include: Gaseous diffusion, Centrifuge Separation, Electromagnetic separation, Aerodynamic processes, and Laser enrichment.
FAS Resources on Uranium Enrichment:
Uranium Enrichment Countdown Calculator
Uranium SWU Calculator
Uranium Centrifuge Cascade Simulator (beta)