According to the Lighting Research Center, a ballast is an electrical component that limits the amount of current that can flow through a fluorescent light tube. Without a ballast, a fluorescent bulb would draw more and more current until it blew itself out. In commercial applications, fluorescent bulbs and ballasts are often two separate components, while consumer fluorescent lights, such as CFL bulbs, often combine the two units together.
The reason for ballasts in fluorescent lighting is that a fluorescent tube actually has a negative electrical resistance coefficient. Since the electrical current of a circuit consists of the voltage divided by the resistance, as the resistance falls, the current increases. Without a ballast to regulate the current, it would increase until either the circuit failed or the fluorescent lamp self-destructed. In consumer CFL bulbs, the ballast often contains a built-in end of life feature to destroy the electrical circuit, preventing a possible ballast failure from causing damage or a fire. Some ballasts are mechanical, using a copper coil wound around a magnetic core to alter the electrical properties of the circuit. Others are electronic, using solid-state circuitry to achieve the same ends. Many high-end fluorescent ballast combine both methods in order to provide maximum current protection.