Electronic cigarette batteries come in various different sizes and colours. Generally speaking, the larger the battery, the longer it will hold a charge.
Some batteries output a constant voltage of 3.7v (like the mini eGo battery 400mAh), and on others the voltage can vary. The standard eGo 650mAh / 900mAh / 1100mAh battery outputs 4.2v when fully charged and starts to decrease through use. When these eGo batteries drop below 3.7v, the power cuts out and needs to be recharged.
Variable voltage batteries allow you to change the voltage that they output by turning a dial (if an ego spinner), or by buttons (if on a more advanced kit). The benefit of a VV battery means that the user can select the perfect voltage for their clearomiser resistance / personal taste (this is called a sweet spot).