Distilled water is water that has been through a “still”…that is water that has been boiled and the vapors condensed…resulting in water with nothing in it.
Reverse osmosis is a process where they force water under pressure through a membrane that only lets water molecules through it, holding back everything else…resulting in water with nothing in it.
In freshman level college chemistry I asked if there was any difference between the two and I was told that only the most sensitive tests stretched to their very limits can distinguish between the two.
You should only use distilled or de-ionized water in your car's radiator...this will prevent mineral build up.