The most common grow-light is LED. However, I am not a terrific fan of LED grow lights. There's a lot of flim-flam out there, and subjective reviews. It's hard to know what is a decent light. It seems like LED grow light fixtures are disposable too. You can't just replace the diodes as they incur "lumen deprecation." (However, there are some DIY solutions which look good. I exited this topic around 2015.).
I like to use household LED lightbulbs and, traditional grow lights. I like these because they're commoditized, componetized, standardized. You can replace parts easily. Lightbulbs cost $1 each, and use standardized mounting hardware. Traditional grow lights can have their bulbs and ballasts replaced.
Traditional Grow Lights
Ceramic Metal Halide (CMH)
I like to use CMH for 7-foot tall tents at 35w/sq ft (not counting the ballast). A LEC 315 covers a 3x3-foot tent. I use it in a 4x4 tent with supplemental lighting around the 6-inch perimeter to maintain 35w/sq ft.
I like remote-mount ballast hoods in warm climates to keep some of the heat out of the tent. Air-cooled hoods are available to extract the tent's air through and even cooler. I haven't used one. The heat doesn't seem that high, and the glass-enclosed hood would reduce the light efficiency (to some extent). I don't think it would be worth it.
An air-cooled hood could be used without the glass front. It would draw air from the vicinity of the bulb. That might help (without light loss). But, the carbon filter would have to be outside the tent (pushing air though it at the end of the duct). A dust filter would be needed to protect the carbon filter from dust. (When drawing air into the carbon filter, a dust filter covers the outside of the carbon filter. It catches a lot of dust. I wouldn't push air into the filter without something to minimize that dust.).
Replace the bulb every 3-4 grows. There are 3000k and 4000K bulbs available. Vegging under 3000k produces more compact plants (tighter nodes). If you use 3000k for both veg and flower, you can rotate your flower bulb to veg (discarding the older veg bulb).
I like T5HO for shorter spaces at 40w/sq ft. I like to use 2-tube fixtures so I can cantilever the light around the canopy. For example, a 4x2-foot tent needs six 54-watt tubes (4-foot long). You can buy a six-tube fixture. But, I prefer 3 two-tube fixtures so I can move the lights further to the sides, for better coverage.
The ballasts can be replaced with remote-mountable ballasts which will remove some heat from the tent.
In veg I use three cool (6500k) for one warm (2700k) tubes. In mid-flower, I will do two cool to one warm. In late flower I will do one cool to one warm.
I replace the flower bulbs every two grows, and rotate them into veg use (discarding older veg lights).
 Fulham "Racehorse" RHA-UNV-254-LT (for two tubes), RHA-UNV-454-LT5 (for two, three or four tubes). Or, "Longhorse" LH5-120-L (for two tubes). These ballasts can be mounted up to 18 feet away.
The ballasts produce a high voltage. Use a common HID (HPS, MH, CMH) ballast-to-hood extension cord to wire the remote ballast to the fixture. Cut the connectors off the ends, and wire it directly. Cut the cord in half, and use a heavy-duty, twist-lock, high-voltage plug (male and female) to connect the cut wires in a way that allows the fixture and remote ballast to be easily uncoupled.
Household LED Lightbulbs
This PDF document discusses why I like Household LED lightbulbs.
Summary: they're cheap (15 cents per watt); easily available/replaceable. Unlike old CFLs, they're directional by design (just cut off the plastic diffusion globe).
Something I really like: by distributing the watts around the plant (compared to all on top) you can put the light closer to the plant. The way the inverse-square law works, each time the light's distance doubles, the amount of light reaching the surface is reduced to 1/4th. Therefore, if you can put 4 bulbs around the sides of a plant, that's an enormous efficiency compared to the light traveling from above the plant. If you had a lightbulb 6" from the lower-side of the plant, and moved it above the plant, the distance to that lower part of the plant could be 18". That would reduce the amount of light reaching that spot to 1/4th (and then 1/4th again, or 1/16th).
It can be a lot of work rigging up the sockets, and mounting/positioning/aiming all those lightbulbs. But, it's a huge efficiency. I grew a plant from veg to harvest using lightbulbs. I had large, dense buds. I flowered at only 18-22w/sq ft.
UPDATE: This PDF was written in 2015. It needs to be updated. Since then I've tested Ecosmart bulbs from Home Depot (the cheap, non-dimmable brown-box). GE "basic" LED from Lowes. They're all pretty much the same.
The newer PAR 20, 30, 38 (and BR30) floodlights look very useful. You can cut/pry the front lens off. They typically have another lens over the LED diodes. You can remove that. Then, the floodlight is essentially the same as a lightbulb (i.e., the diodes are surface-mounted like lightbulbs, directional by design.). With the additional benefit of a built-in reflector.
Today, LED lightbulbs seem to be going back to frosted-glass (with LED "filaments" suspended within the round part). For growing, that's a step backwards. You want to use LED lightbulbs with a base (like those shown in the PDF) with the LEDs surface mounted. If this type of lightbulb is replaced by the "filament" type, then the only useful household LED lightbulb will be the PAR 20, 30, 38 (BR30) floodlights (lenses removed).
Mounting, Reflecting & Powering Household LED Lightbulbs
The lightbulbs are easy to acquire. The hard part is mounting them in an easy, flexible manner. This PDF contains a variety of things I've found.
Sometimes, all you need to start having ideas is to see some of these things.
Tent-leg Sidelight Mounts
This PDF shows how to attach clamp-on reflectors (from the hardware store) to your tent legs. These slide up/down the tent legs, and have a swivel mount which allows you to aim the lights.
Tubular Top-Mount Fixture for LED Lightbulbs
This PDF shows a "fixture" I made for shorter tents. You can move the lights out to the edge and point them back toward the center. Requires minimal vertical space to use.
Folding-Arm Tent Leg Mount
This PDF shows a merging of ideas (the tent-leg mount & the flexible "fixture's" folding arm). This mount gives you some "reach" into the center of the tent (good for seedlings. The original mount is better when the plant is mature, and you need the light as recessed into the corner as far as possible.).