Shortfills are e-liquids that come in larger bottles, usually around 50ml and 100ml bottles. They're called "shortfills" because you need to add your own nicotine shot (or "booster") to make them up to a higher strength. Shortfills are designed for sub-ohm vaping, producing huge clouds of vapour.
Nic salt e-liquids are made with a certain type of nicotine which is absorbed into the body more quickly. They differ from freebase e-liquids as they provide a smoother vape even at higher nicotine strengths.