Double Ended Lights/Bulbs:
Double ended lights are lighting fixtures that take a special bulb that have two points of contact. This varies greatly from the traditional "single ended HPS bulb" that have been industry standard for hydroponics for decades. Double ended lights have been used by industrial and hobbyist growers for about the last decade. The internal components of this type of light make it so that there is greater penetration which results in a higher yield. There are several brands of double ended light fixtures which have greatly decreased in price over the year. Most models are dimmable, which allows the user to adjust how much light and power is used, making it possible to save energy while still getting optimal results. It is also possible to replace the HPS with a different spectrum bulb for different stages of plant growth.
Single Ended Lights/Bulbs:
These fixtures have one point of contact and a standard E40 Edison screw thread, which is an enlarged version of your home light fixture. These types of lights emit less heat than the double ended bulb but are also less energy efficient as they produce less light density.
Single ended light fixtures can also be adjusted for wattage; however, there are less options for light spectrums than the double ended fixture. Single ended fixtures cost significantly less to set up than their double ended counterparts but offer less options for customization of light spectrum for different growth stages.
LED lighting is a fantastic option for avoiding heat issues that come with single ended and double ended lighting fixtures (High Intensity Discharge lights or HID light). NOT ALL LEDS ARE CREATED EQUALLY. There will be plenty of extraordinarily cheap options on the internet, but not all of these options cover the invisible (to the human eye) spectrum that is important when replicating sunlight indoors.
The light quality is comparable to that of HID lights. However the light penetration sometimes decreases with LED lights, as they will not burn as bright as an HID.
LED bulbs also have an incredibly long lifespan compared to HID lighting.