How does a Uranium gas centrifuge work?

There are two main isotopes of Uranium, U-238 and U-235.  U-235 has a shorter half life, which means it will die young, well, younger than U-238.  This makes it very active, just like you would be if you found out you only have 2 weeks to live.  This increased activity makes it hard to hold down, which is what centrifuges try to do, hold you down.  Well, U-235 sticks it to the man, or rather centrifuge, and moves to the center while the more lackadaisical U-238 is complacent and allows itself to be held down.

The U-235 which has overcome the oppressive forces of the man, er, centrifuge spinning at up to 100,000 rpm, is then removed and placed in a new centrifuge.  This process is repeated numerous times.  After this cascade of purification the concentration of U-235 has increased from about 0.7 percent in unprocessed ore, to about 4% for power plant operations; or up to 90% for weapons applications. 

Those isotopes lucky enough to be chosen for the weapons of mass destruction then have the opportunity to rot away in forgotten silos and never be used, and the brochures made it sound so promising and fun.  They will never get the chance to fulfill their dream and go out with a bang; with the possible exception of Iranian U-235, those are the lucky isotopes.