Distilled water is tap water that has been boiled and condensed, so everything the tap water was carrying has been left behind in the boiler. The difference is easiest to understand not as chemistry but as residue: distilled water leaves nothing behind when it evaporates, and tap water leaves the white crust you already know from your kettle.
The process, and what stays behind
In a distiller, water is heated until it boils, the vapor rises and is condensed against a cooled surface, and the liquid is collected. Dissolved minerals, salts and metals cannot travel as vapor, so they remain in the boiling chamber as scale, which is why the chamber of a working distiller has to be descaled regularly. Tap water arrives with those minerals still in it, along with a disinfectant residual and hardness picked up from the geology of its source.
Where the difference shows up in your house
Anywhere water evaporates or boils, the minerals it carried are left behind. That is the white crust inside the kettle, the fine dust an ultrasonic humidifier puts on the furniture, the flakes a steam iron spits onto a shirt, and the scale inside a water heater. Distilled water does none of that, which is why appliance manuals reach for it. On the other hand, every fixture and appliance in the house runs on tap water, and no jug of distilled water changes that. Hardness at that scale is a softener question.
What your tap water actually carries
You do not have to guess. Every community water system publishes an annual Consumer Confidence Report covering its source, its treatment and what was detected, and the EPA maintains a finder for those reports. That is the document that tells you whether the taste you dislike is chlorine or chloramine, and how hard your water is. On a private well nobody produces that report, and a lab panel is the only equivalent. Our well water testing guide sets out the parameters worth ordering.
Drinking one versus the other
Tap water from a compliant community system is treated to be safe to drink, and it tastes of the minerals and the disinfectant it carries. Distilled water is safe too, and tastes flat because it carries nothing. Neither is a health upgrade over the other for a person eating a normal diet. Where distilled water genuinely helps is when a specific contaminant is present and untreated, and even then a filter aimed at that contaminant is usually a better answer than jugs.
The cheaper way to get the benefit
Most people asking this question want water that tastes better and does not scale things. Those are two different fixes. Better taste comes from carbon filtration at the tap, which removes the chlorine responsible for most complaints. Less scale comes from softening at the point of entry, which stops the minerals reaching your heater and fixtures in the first place. Buying distilled water addresses neither of those things beyond the jug in your hand, which is worth knowing before it becomes a standing order.
Questions people ask about what is the difference between distilled water and tap water
Is distilled water just boiled tap water?
No. Boiling kills organisms but leaves the minerals in the pot and concentrates them. Distillation collects the steam and leaves the minerals behind, which is a different result entirely.
Why does my kettle scale but distilled water does not?
Scale is the calcium and magnesium your tap water carried, left behind as the water boils away. Distilled water has already had those minerals removed, so there is nothing left to deposit.
Can I use distilled water in a humidifier?
Yes, and it is usually recommended, because it stops the white mineral dust that ultrasonic units otherwise spread around the room. Check the manual, since a few models specify particular water types.
Is tap water tested more than bottled distilled water?
Community tap water is monitored continuously and reported annually through the Consumer Confidence Report, which the EPA helps you find. That level of public reporting has no direct bottled equivalent.