4 Stage Reverse Osmosis System

[Bjm]

Hi
4 Stage Reverse Osmosis System, Can anyone explain these to me please?

Am i right i thinking that these units can make sea water drinkable? and i notice there are many stages of this unit, what is the recomended minimum for say water from a lake or a stream, to make it ok? I also notice that some need a pump, and some not?

When i store water, would it make any differance if i filter it first?

I have seen these for sale boasting 75 gallons a day of clean water. so was thinking whats the catch?