Light is essential for plant life. In general terms, the more
light you have, the better your harvest will be. However, like
most things in life, there will be compromises involved. In my
case, I had to take two facts into consideration: 1) I live in an old
building, and power is at a premium...too much power going to my lights
would result in tripped circuit breakers, and 2) In the summer, the
interior of my apartment sometimes exceeds 100 degrees. It's a
fact of life that lights generate heat.
Personally, for my casual growing purposes, I chose CFL lights. They are inexpensive, they last a long time, they don't use much electricity, and the lower wattages put out virtually no heat...the last two being particularly relevant to my situation.
If you choose to go the CFL route, I recommend you get three fixtures (I like the standard reflectors myself) and four bulbs. Get two bulbs as close to 2700° K as you can. Get two more bulbs as close to 6400° K as possible. Use two 6400° K bulbs and one 2700° K bulb through the vegetation phase. Switch to two 2700° K bulbs and one 6400° K bulb when you change to flowering phase.
There are of course, more powerful and more expensive lighting solutions including Metal Halide lamps, High Pressure Sodium lamps and LED lamps. Due to the restrictions of my antiquated home wiring, I have not been able to personally try and of these alternative light sources.
Two other essentials you will need as part of your lighting system are 1) a way to raise your lights as your plants grow taller and 2) a timer to switch from vegetative to flower phases. In my case, I looped my lamp cords over the top of the metal frame of my grow tent, then raised the light to the proper height and locked it in place with a clothes pin. I've seen two recommendations for lighting for the vegetation phase: leave them on for 18 hours a day and leave them on for 24 hours a day. I've gone with the 24 hour cycle for the vegetative phase myself. Everything I've read, recommends 12 hours on/12 hours off once you go to the flowering phase.
If my apartment were cooler (so that the extra heat put out by Metal Halide or High Pressure Sodium bulbs would not be a problem) and my electricl system rigorous enough to handle the extra wattage, I would definitely upgrade from my CFL lights. The extra light will grow bigger and denser buds.
However, if you're on a budget, or on the verge of exceeding the capabilities of your electrical system, Compact Fluorescent Lamps work just fine. Our personal choice is Stealth Hydroponics Dual Spectrum, High Ouput CFL kit: Four fixtures and four bulbs will run you around $120.
Full Disclosure: I personally paid for my Stealth Hydroponics CFL lights. I've tried several of their lighting selections and the above kit gives the biggest bang for the buck. I also get a slight commission if you click the above link and purchase my recommended lights.