Compact Flourecent Light (CFL) is a form of lighting that is widely used as indoor grow lights for hydroponic growers, domestic to commercial. Mainly used for propagation.
CFL's cost less than most other types of hydroponic lighting. They also produce less heat which makes them a good choice for getting your plants started, sprouting seed, and growing clones. They are also a good option for growers who have smaller space to grow in. In short they are good for getting started without breaking the bank.
CFL bulbs come in a range of types, most common is the spiral CFLs (which look like larger versions of the CFL's you may be using at home.), and T5 fluorescent tube lighting, which are long straight tubes.
Being cheaper and emitting little heat does have its drawbacks. Mature plants will have slower growth rates under CFLs as opposed to other types of hydroponic lighting tech. For this reason, hydroponic growers will use CFLs mostly for baby plants and clones. However, for hobby growers looking to grow a modest amount on a small budget, CFL lights may very well be the best choice.