Ego style batteries or basic mods with voltage control. Model names such as "Twist" or "Spinner" refer to the adjustable dial at the base of Ego style batteries. They generally dont come with a charger.
Variable voltage (VV) allows you to adjust the vape to a level that suits you. Higher voltage tends to offer a warmer more fuller vape but too high and you risk burning the juice. Your battery will flatten more quickly at higher voltages.