@bombastinator Is spot on. To add to what he said, Salt nicotine is formulated a bit differently than traditional nicotine (Freebase Nicotine/Normal Nicotine) in which organic compounds are added to make this nicotine get absorbed into the bloodstream a lot faster than normal nicotine would.
Nicotine salts also have a lower pH level which means it has less of a throat hit compared to normal nicotine and this therefore means you can go higher in strengths without getting a strong throat hit. It does have its benefit since you can now do lets say 35mg and get your nic fix faster without the harshness.
As
@bombastinator has pointed out, Juul started introducing Nicotine Salts in their pod devices and ever since then it has gained popularity and has been abused by teenagers. South Park even did an episode on vaping and teenagers. I think it was Season 22 Episode 04.
With all this said, I would say stick to normal nicotine unless you really crave cigarettes and need a higher nicotine fix. In this case, I recommend getting a decent pod device and some salt nicotine e-liquid.