PC (power compact) lights = prob. the cheapest light fixture you can buy for keeping saltwater corals except you are very limited to what you can keep esp. in a deep tank. it runs cool comparitively speaking but bulb replacement is expensive and bulbs comparitively speaking prob. have the shortest life span
T5 lights = T just stands for tubular and 5 stands for the size of the bulb. a T5 is smaller in diameter then say a T8 or T12 (which really are only good for actinics, ill explain more further down) T5s are "medium priced" and run hotter then PCs but cooler then halides. HO stands for High Output. You can keep a decent amount of corals running t5s but again in a deep tank things towards the sand bed gets difficult. i run t5s and love them. when you have them set up right with 4,6,8 (i know someone that has 16 bulbs with 4 ballasts over a 265 lol) its easier to switch out your color preference. bulbs are about $20-25 a pop but switching out atleast 4 a year = $100 yearly.
Metal Halides = can keep just about any corals/clams even on the sand bed depending on wattage. they generally come in 75, 150, 250, 400 and 1000 watt setups. halides run the hottest out of all the lights and in alot of cases you will need a chiller to keep your tank cool. more expensive to purchase then PCs and T5s. Usually people run halides AND T8 or T12 and even T5s or PCs with actinic bulbs to get the "blue coloration"
LEDs = this is a newer technology and at first ive heard only great things esp about the soloaris. i hear strips of LEDs are failing in peoples fixtures. recently ive been hearing that its garbage and theyre even going out of business esp. because some wisconsin company purchased the patent for using LEDs or something along those lines. The initial cost of a fixture of LEDs is very high but it runs cooler and more energy efficient.
Retrofit = this is the DIY light fixture. basically it involves you buying endcaps, bulbs, ballasts and building a hood to mount the lights in. if you have DIY skills ( even if you dont they sell kits that are fairly simple ) this is the way to go as you'll prob. get the most bang for your buck. i retrofitted 4 t5s and very soon ill be adding an additional 2 with the other ballast i picked up. retrofitting doesnt look as pretty as the store bought pre built fixtures enclosed in metal but going this route can save some greenbacks.
Ballast = this is the powerhouse of the bulbs. without a ballast the bulbs are basically useless.
Bulbs = actinic bulbs really are just high in blues and cause corals to "pop" and appear neon and colorful. 50/50 bulbs are half actinic and half daylight. 75/25 bulbs are 75% actinic 25 daylight. kelvin is very important too. 6500K will give corals the best growth (as well as algae) but will make corals appear brown and ugly. 10,000 is a decent medium as it provides decent growth and ok colors, then theres 12, 14, 20ks the higher you go the slower corals grow but the better they look. color choice is really personal preference. if using t5s i suggest a mixture of different kinds of bulbs.
IMO i think the 4 watt per gallon, 5wpg, 6wpg and all these "rules" are not rules. simply its just something to aim for, you have to keep in mind that all different colors prefer different amounts of light and flow. some like lots of ligth and low flow while others might like little to no light and heavy flow (which tends to be things that feed only from the water)
another thing to remember is when it comes to any saltwater tank a protein skimmer is most important. when it comes to reefing the most important things are prob. the skimmer, lights and flow. im sure more people have things to add and disagree on but heres my input. hope it helps for a better understanding.