Type of Lighting
There are two basic types of lighting used for reef aquariums; metal halide and fluorescents.
Metal halide light fixtures can have either single-ended or double-ended lamps (bulbs). There are three basic kinds of metal halide ballasts; electronic, standard magnetic, and HQI magnetic. Electronic ballasts tend to use less electricity, but might also under drive the bulb (the bulb doesn't produce as much light as it could). Standard magnetic ballasts are generally better for single-ended bulbs while HQI magnetic ballasts are intended more for double-ended bulb fixtures. However, these are all generalized statements. Each combination of bulbs and ballasts will have their own pros and cons.
Standard fluorescent lights are not usually enough to satisfy the exacting lighting demands of most reef aquariums. Therefore, most reef aquarists will use VHO (very high output) or T5 (narrow-diameter) fluorescent lights which provide several times more light than comparable standard fluorescents.
Wattage
Common wattages for metal halides are 100w, 150w, 175w, 250w and 400w. Wattage for VHO and T5 lighting varies with the type and length of the bulb. Ignore rules of thumb when it comes to determining how much wattage you want over your tank. How much light you will need will depend on what kinds of corals you plan to keep. Aquarists who would like to keep corals with intense lighting needs (such as Acropora), should favor metal halide lighting. Aquarists keeping mostly soft corals with low lighting needs (such as leathers), can easily get by with quality VHO or T5 fluorescent lights.
To help choose a metal halide light, consider the length of your tank. Place one metal halide light fixture every 2-3ft of horizontal length. If your tank is 1 to 2ft in height, 100-175w doubled ended (or 150 to 250w) lamps should be plenty of light for any type of coral. If your tank is 2-3ft deep, 250w double ended or 400w single ended lamps should be plenty. However, keep in mind that there are no hard and fast rules for choosing aquarium lighting. Look at the lighting choices of your fellow aquarists and talk to people at your local aquarium store to get advice and ideas about different lighting options.
Temperature (Kelvin, K)
Lamps/bulbs produce light of varying wavelengths. Different wavelengths produce different colors of light. The more blue light a lamp produces, the higher its Kelvin, K rating. This is also called the "temperature" of the light. In lighting, lower Kelvin lights are described as being warm (more red) with higher Kelvin ones being described as cold lights.
Blue light penetrates water deeper than other colors. Therefore, a bulb of a higher K value will produce more light that can penetrate deeper into water. However, since bluer light takes more energy to produce, higher K bulbs of the same wattage will produce less light of other colors (wavelengths).
Actinics
Actinic lighting is lighting that produces mostly blue light. Many aquarists add actinic fluorescent lights for aesthetic reasons (or to add more blue light to their broad spectrum lighting). They are also sometimes used to created a dawn and dust type lighting before and after the main lights are on.