milli-q water is made by passing the source water through mixed bed ion exchange and organics (activated charcoal) cartridges. the source does not have to be distilled or deionized (reverse osmosis) but the cartridge life will be shorter if it is not. finally the water is passed through a filter which should pull out any intact organisms. sometimes there is a uv irradiator attached, as well.
milli-q is a brand name, there are other brands that perform as well as the milli-q system.
-mdfenko-