A centrifuge is a piece of equipment, generally driven by a motor, that puts an object in rotation around a fixed axis, applying force perpendicular to the axis. The centrifuge works using the sedimentation principle, where the centripetal acceleration is used to separate substances of greater and lesser density. There are many different kinds of centrifuges, including those for very specialised purposes. It can be used for viable counts, when shaking the culture e.g. yeast, out of suspension.

It is also what we use in the lab to spin the blood to separate the serum from the plasma. *oops just caught on to the category. Please ignore my dumb answer*

http://en.wikipedia.org/wiki/Centrifuge

The answers above are right. They are especially useful for making enriched nuclear material because by spinning a mixture the heavy stuff settles to the bottom. The more highly radioactive isotopes are heavier than their nondoped brethren. So extremely high (read: can't get them on Ebay) quality centrifuges are needed to separate the the doped plutonium from the regs.

What you're talking about (going by the category) are Gas Centrifuges to Enrich Uranium. There are two main ways to refine Uranium and it requires the Uranium be in a gas state for both. Uranium-235 (what you want) and Uranium-238 are both chemically identical, with one of the only differences being that U-238 is slightly heavier (on account of 3 more neutrons per atom) When Uranium comes from the mines it is a yellow powder (uranium oxide) called "yellow cake". There are only about 7 atoms of U-235 per thousand uranium atoms in yellow cake on average so this is why you need to "enrich it" to get the amount of U-235 up. Commercial reactors use about 4% concentration (where 4% of the uranium they use is composed of U-235). This is far off being what you need for a weapon (greater than 90%) so you can't just take reactor fuel and use it in a bomb, you just have to enrich it even more first. You firstly convert the uranium oxide into a gas (Uranium Hexafluoride) by heating it and converting it in a chemical plant called a "conversion plant". This gas is extremely reactive and corrosive, so much so that special materials are needed in the enrichment equipment and the area must be kept clean (eg of oil and grease) to stop undesired chemical reactions. By using gas centrifuges you're relying on the small mass differences and the rapidly spinning centrifuge to separate out the two isotopes. The centrifuge is basically a large drum that rotates at very high speeds, so that U-238 concentrates on the outer edges of this drum with the U-235 concentrated near the central axis. The trick is that the U-235 is made to flow the opposite way to the U-238 and then gets bled off. This only slighly increases the concentration of U-235 however each time it's done. So for large scale enrichment countries use a large number of the centrifuges working in a cascade (each feeding into the next one and so on) to achieve a good output of enriched Uranium. Gas Centrifuges are very demanding to construct due to their special requirements (high tensile strength, corrosion resistance etc), but they are cheaper to run than any other enrichment method. They also offer good expansion possibilities as you can bolt more stages on as you need to. The other two main methods are Gas Diffusion (uses the same gas as above) and Calutrons (ionises uranium and uses magnetic fields). I only mention them here for completeness. These methods are used for Uranium. Plutonium production needs an entirely different approach (due to it being man made).