Reverse Osmoses.

[Alan Wheeler]

So what exactly is meant by the term "Reverse Osmosis" From what I have so far seen, a water maker simply pumps water through a series of ever decreasing particle size filters, till everything other than pure water is filtered out. Not kinda what I thought Osmosis was or reverse Osmoses for that matter.
Or are there two or more different types. If yes, then what type is better.
One thing I here in most all cases, is that the filters on water makers are expensive and don't last long in dirty water. But so far, this is the fastest means of making large quatities of water.