Nicotine salt is created by pairing regular nicotine with (usually) benzoic acid. This changes the formulation and PH of the nicotine allowing it to absorb into your bloodstream quicker and deliver a nicotine hit quicker. Regular e-liquid, known as freebase, contains the original type of nicotine. E-liquid that contains freebase nicotine is usually lower in concentration. This is because the nicotine hit takes a bit longer to reach your bloodstream so it’s easier to overdo it. Also, freebase nicotine gives a stronger throat hit than nicotine salt, meaning freebase liquids are much harsher on the throat than a nicotine salt based vape juice.