The salt used in a water softener is not the same as the salt we eat — and no chemicals are added either, as is sometimes claimed. The difference: all salt starts out as dried sea salt with minerals. Salt for a water softener is purified salt without those minerals — without iodine, for example — because those minerals can disrupt the softening process. We only use purified salt of professional quality, with no additives. Stories about salt that supposedly doesn't clump are mainly a marketing trick to sell a more expensive product.
There are two kinds of water softener: mechanical and electronic. An electronic softener measures when the water needs treating, and preferably regenerates at times when you're asleep. A mechanical softener works on a timer, not on demand — if you're on holiday, you run the risk of bacterial growth because the system doesn't regenerate without demand. Conversely, a mechanical system can also start regenerating at a moment you don't want it to, without you having any say in it.
Curious which size water softener suits your household? Take a look at the ZeNO product page for the options and an explanation per size.