An electronic cigarette aims to provide nicotine in vapor form with a mix of ingredients that provide flavor and act as soothing agents. There are three main ingredient types:
2) Active ingredients
These together provide the ecig vapor with all its characteristics namely: taste, tang, smell, vapor density and even its aftertaste. There are more subtle effects such as throat hit, dryness(rather the lack of it) and even how long the cartridge lasts.
Base: Propylene Glycol or Vegetable Glycerin. Both are FDA approved ingredients (yes, even Propylene Glycol is FDA approved)
Base: Food-grade Glycerol
Base: Distilled water
Active ingredient: Nicotine
Flavor: Citric Acid
This is pretty much standard across most ecig brands with the exception of citric acid. But then what makes one electric cigarette cartridge better than others? It is definitely due to e-liquid related factors such as quality ingredients and achieving the right concentration but also due to cartridge design and they way that the cartomizer fits the battery.
Some companies also add ingredients such as alcohol and various oils. E-juice is not at all expensive so usually reputable companies need not skimp on quality ingredients. But some do. For instance using alcohol instead of Propylene Glycol or Glycerin reduces shelf life significantly in exchange for very little cost savings. These companies pay much higher in the longer term in the form of expensive advertising, lost customers and cartridges that make it sooner to landfills.