Plants require ultraviolet light in order to grow. When you grow plants outside, they get sufficient light from the sun, but indoor plants benefit from supplemental ultraviolet light--even if you place them near windows. Regular fluorescent lights emit low levels of ultraviolet light and can be used for small indoor growing setups. For larger setups, use high intensity discharge bulbs.
Regular Fluorescent Lights
If you only plan to grow a few small plants, use regular fluorescent bulbs to supply light. These bulbs cost less than high intensity discharge bulbs, but don't emit as much light. Fluorescent lights come in both four foot long tubes and small compact sizes. The compact bulbs actually emit approximately twice as much light as the larger tubes, and are not significantly more expensive. In most cases they should be used. Only consider tubes if you need to spread the light more evenly over a large area.
Regular fluorescent lights come in three different types, each of which emits a different spectrum of light. Cool white bulbs mostly contain "cooler" light, meaning light on the blue end of the spectrum. Warm white bulbs mostly contain "warmer" light, which is on the red end of the spectrum. Full spectrum bulbs emit light evenly across the spectrum. Plants will grow best under the full spectrum of light, so you can either purchase full spectrum bulbs, or a mixture of cool white and warm white bulbs.
Plants depend on warm light more than cool light, so if you are using a mixture of bulbs, use more warm white bulbs. Certain plants prefer a different emphasis of the light color spectrum. You can use a growers guide to discover which color lights your plants need the most, and plan your lighting accordingly. You may come across plants that need more cool light in their early life, but more warm light later on.
High Intensity Discharge Bulbs
If you are growing a large number of plants inside, or you want to grow the best plants possible, use high intensity discharge bulbs instead of regular fluorescent bulbs. These bulbs, known by the acronym HID, emit about five times as much light as compact fluorescent bulbs, but only use two and a half times as much energy. The downside of HID lights is that they are much more expensive than regular fluorescent bulbs. For an amateur hobbyist, the cost may outweigh the benefit.
You can purchase a variety of different types of HIDs. For most hobbyists, the only necessary type is metal halide. These will emit a wide spectrum of light, but have the most intensity in the blue spectrum. They can be used as your only source of light to grow your plants. As you become more involved with the hobby, you may wish to use other types of HIDs to further improve the growth of your plants. These are not necessary, and will cost you more money, but will help you grow the best plants. One example of another HID is high pressure sodium, which will increase the size and number of your flowers.