LED lamps’ internal drivers function as a power supply, taking the power — which is either from the fixture ballast in a replacement LED lamp, or is the ac house power directly wired into the fixture — and convert and regulate it to power the lamp’s internal LEDs. As you can read in the LED definition, LEDs are current-controlled devices, so LED drivers are current regulators. (Most electronics are voltage controlled.)
How does “efficacy” differ from “efficiency”? In the lighting world, efficiency refers to the lighting fixture, which is the device that holds the lamps. Fixtures reflect the light out from the lamp onto the area to be illuminated. No fixture is perfectly efficient and some light is lost due to reflection and absorption. If a fixture has a 100 lm lamps but only emits 70 lm, then it is 70% efficient.
Use the term “efficacy” when the units of the input differs from the units of the output. In lighting, we are interested in the amount of light, measured in lumens, produced by an amount of electrical power, measured in watts. If a lamp consumes 9 W to produce 800 lm, then its efficacy is 89 lm/W.
The metal device, mounted on the ceiling, that holds the lamp(s). In the world of fluorescent lighting, the fixture also has the ballast. When you buy a fluorescent light fixture at a store such as Home Depot, the ballast is always included as part of the fixture.