One of the biggest advantages to indoor gardening is the regulation of plant growth through the use of artificial sunlight. The most commonly used grow lights are metal halide, high pressure sodium, and fluorescents. Each has its advantages that I will briefly cover. Both metal halide and high pressure sodium lights are high intensity discharge, which mean the light they produce has a better ability to penetrate the plant leaves.
Fluorescent light can grow plants indoors, but produces much smaller plants and is generally considered to be far inferior to both metal halide and high pressure sodium lights. Fluorescent lights are most often used with seedlings or clones, as the light produced is gentler on fragile plants.
It is a common myth that plants can't utilize more than 12 hours of light per day and require a dark period. It isn't true. Try to imagine a plant's growth cycle is similar to an animal, the more you feed it, the fatter and bigger it gets. If you never decided to switch to 12 hours of darkness your plants would continue to grow until they reached an unmanageable size!