units usually have 3 stages or more.
The first stage is usually a sediment prefilter to rid your water of large particles. (these particles are still rather small to the human eye).
The second stage is usually a carbon filter that will remove the clorine and other chemical components of the water. (most of those britta style filters are basicly just carbon filters)
The third stage is usually the RO
membrane. This is where the water is pushed thru at rather decent pressure the membrane. only the smallest of particles can get thru and ideally what you get comming out of the RO
membrane is nearly pure H2O.
The fourth stage if you have one is usually composed of a DI
membrane. This membrane will in effect remove any rouge particles that might have managed to get thru the other previous three stages. (only RO
filters have this DI