Distilled water is condensed steam and reverse osmosis water is water pushed through a membrane, and for most household purposes the two are interchangeable. The differences only start to matter when a specific appliance or task is fussy, so the useful way to answer this question is job by job rather than in the abstract.
The mechanical difference in one paragraph
A distiller boils water and condenses the vapor, so anything that does not travel with steam stays behind in the boiler. A reverse osmosis system forces water against a membrane at pressure, so anything the membrane rejects leaves with the reject stream to the drain. Both end up with water carrying very little dissolved content. They fail differently at the edges: volatile compounds can carry over in a distiller without a carbon stage, while a reverse osmosis membrane depends on its prefilters to survive chlorine.
Where the manufacturer says distilled, use distilled
Steam irons, certain humidifiers, some medical devices and lead acid batteries are commonly specified for distilled water. Where the instruction manual names it, follow it rather than substituting, because the specification usually reflects testing on residual mineral content and sometimes on warranty terms. This is not a claim that reverse osmosis water would damage the device. It is a recognition that arguing with a warranty over water is a poor trade for the price of a jug.
Drinking, cooking and coffee
For everything you consume, the two are effectively equivalent and both taste flat, because the dissolved minerals that give water flavor have been removed. Coffee is the interesting exception in the other direction: specialty brewers often want some mineral content for extraction, which is why remineralization cartridges exist on reverse osmosis systems and why fully demineralized water is not automatically the best brewing water. For ordinary household drinking and cooking, choose on convenience rather than chemistry.
Humidifiers, CPAP tanks and steam appliances
Anything that boils or atomizes water and leaves the residue behind benefits from low mineral water, which is why humidifiers stop crusting and irons stop spitting white flakes. Both types qualify. Where a device manual specifies distilled, use it. Where it simply asks for demineralized or low mineral water, reverse osmosis water is a reasonable and much cheaper choice if you already have a system, since you produce it at the tap instead of carrying jugs home.
The practical decision for a household
If you need small quantities occasionally, buy distilled by the jug and stop thinking about it. If you need water daily for drinking and cooking, a plumbed reverse osmosis system produces it continuously with no carrying and no storage, and that is what most households actually want. A countertop distiller makes sense where plumbing is impossible or where you want distilled specifically. Our reverse osmosis water filter cost page sets out what the equipment side of that choice looks like.
Questions people ask about difference between distilled and reverse osmosis water
Can I put reverse osmosis water in a steam iron?
Many people do without trouble, but check the manual first. Where the manufacturer names distilled water specifically, following that instruction protects the warranty for the price of a jug.
Is distilled water safe to drink every day?
It is safe, though it tastes flat because the minerals are gone and it contributes nothing to mineral intake. A varied diet supplies those minerals anyway, so the objection is mostly about taste.
Which is better for a humidifier?
Either works far better than tap water, because both leave almost nothing behind to crust the unit or dust the room. Use whichever your humidifier manual permits and whichever is easier for you to obtain.
Does reverse osmosis water leave scale?
Very little, because most of the minerals that form scale have been rejected by the membrane. That is why it suits kettles, steamers and appliances that would otherwise fur up on hard water.