Nicotine Liquid is a liquid form of nicotine extracted from the tobacco plant. Nicotine is much purer than what is used in tobacco cigarettes which uses dried tobacco leaves. Nicotine Liquid is known as Freebase Nicotine but Nicotine Salts are fast becoming popular. 

Nicotine Salts is derived from the same tobacco plant but is extracted by a different method. It was sometimes used to refer to E-liquids in general but its important to note that not all E-Liquids contain nicotine.