Compact Floruscent light. No they are not required for every grow, they are a cheap subsitute handy for growers who grow about 3-4 plants and cant afford HPS and HID.

However many growers with HPS use them in addition because you can position them nicely so the light goes exactly where you want it too. Also they come in alot of light spectrums so if one has a HPS and would like extra blue light for flower they're handy.

The best aspect is its a florosecent but it screws into regular light sockets.

Keep plants at least a inch away, but make sure the lights are within 4 inches of where you want the light to go.