@bombastinator has covered what they are, so I'll focus on your other questions.
Who should use them:
People who can't satisfy their nicotine addiction with freebase nicotine. If you're still having nicotine cravings after increasing your freebase nicotine levels as much as you can without coughing a lung up, nic salts are definitely worth looking at.
Who should not use them:
Anyone not already addicted to nicotine and anyone who's managing well with freebase nicotine. High levels of nic salts are probably more addictive than lower levels of freebase, so there's no point going there if you're not trying to stop smoking. Also, as bombastinator said, we have even less information about nic salts than about freebase nic, so it's better to stick to freebase if it works for you. If it doesn't, we're back to the fact that nic salts are very unlikely to be worse than cigarettes, so why not try?
Where to buy:
It's quite widely available now. Many places that sell ejuice or DIY supplies offer both freebase and salt nic, so if the shop you're looking at doesn't, just try the next one.