The difference comes down to dissolved minerals. Hard water carries calcium and magnesium picked up as water moves through limestone and rock. Soft water has little of either. Hardness is measured in grains per gallon or parts per million, where 1 gpg equals about 17 ppm. Water under 1 gpg is soft, 3.5-7 is moderately hard, and anything above 10.5 gpg is very hard. Most of Arizona sits well into the very hard range, among the hardest in the country.
Hard water isn't a health hazard, but it's hard on your home. The calcium and magnesium precipitate out as scale, that chalky white crust on faucets, shower glass, and inside pipes and water heaters. Scale insulates heating elements, so a scaled water heater works harder and costs more to run. It clogs aerators and showerheads, shortens the life of dishwashers and washing machines, and leaves spots on dishes and film on skin and hair. In Arizona, these effects show up fast.
Soft water comes from a softener that uses ion exchange. Water passes through a resin bed where sodium ions trade places with the calcium and magnesium, which stay behind on the resin. Now and then the system regenerates by flushing the resin with brine from a salt tank. The trade-off is a small amount of added sodium plus the salt and water used during regeneration. Salt-free conditioners don't remove hardness. They change how minerals crystallize to cut down on scale, with more limited results.
You can usually tell hard from soft water without a meter. Hard water leaves spots, fights you when you try to lather soap, and builds scale. Soft water lathers easy, rinses clean, and can feel a little slippery because the soap rinses all the way off your skin. A simple test kit or a free water test confirms your exact grain count. In Arizona, testing matters because levels swing between cities and well water, and it tells you how much softening capacity you actually need.
For most Arizona homes, softening is worth it for appliance lifespan alone. Protecting a water heater, dishwasher, and plumbing from 16-20 gpg scale often pays for the softener over its life, and a lot of homeowners pair it with reverse osmosis for drinking water. If sodium intake or environmental concerns rule out a salt softener, a template-assisted crystallization conditioner is the common alternative, though it reduces hardness rather than removing it.