Use this tag for questions about ballasts used with fluorescent or other types of discharge lighting.
Fluorescent and other discharge-type lights (such as sodium vapor) require a device known as a ballast to provide proper voltages and currents for lamp operation. Without one, the lamp would burn itself up as it has a negative resistance characteristic that causes it to pull more current as the voltage across it drops, or it would simply not light as mains voltage is insufficient to start the electric discharge to begin with.
Ballasts may either use magnetic coils to limit the lamp current and transformers to provide proper voltages for the filaments in a "magnetic ballast", or use high-frequency power electronics for both functions in an "electronic ballast". Modern ballasts also integrate lamp starting functions; older/obsolete types may require an external starter to provide the required voltages for lamp startup.