Nicotine found within the vape liquid is available in two forms; freebase and salt based.
Conventional (freebase) is what you would typically find in your standard e-liquid, it is nicotine in it's purest form making it more potent when heated.
Nic Salts not only contain nicotine but various other organic components, such as Benzoic acid. The addition of benzoic acid reduces the pH level, which means when vaping the liquid is less harsh and in turn allows you to vape at higher strengths.