E-liquid is the liquid substance that is vaporized in an electronic cigarette. Many e-cigarette kits come with already filled cartridges or cartomizers while others have the ability to fill your own. Refilling cartridges with a bottle of e-liquid is much cheaper although it is not as convenient as buying pre-filled cartridges. There are also generally more flavors available in bottled e-liquid form.
Below are links to some of the best e-liquid available: