Ballasts are required to operate many different types of light bulbs.  The light bulbs that require ballasts include metal halides, high pressure sodiums, low pressure sodiums, fluorescents, and compact fluorescents.  Ballast also come in many shapes and sizes like f-can, core & coil, postline, and small compact sizes.  Ballasts are used in commercial, and residential applications when the light bulb used requires a ballast to operate it.