A lamp holder is a light socket that houses a light bulb, typically with threaded sockets. The lamp holder acts as a base which links the bulb to the electrical wiring.
A lamp adapter conjoins normally incompatible lamp parts. They are useful in that the adapters can retrofit older fixtures to fit new bulbs. For instance, a bulb with a mogul base could attach to a socket reducer to fit with a medium socket.
Lamp holders include 250W, 660W, and uno threaded lamp holders. Uno threaded lamp holders are a piece of a full lamp which attaches to a lamp stem and holds the light bulb. Uno threaded holders also provide power and a switch to the bulb.
Some of the lamp adapters available include GU24, E26, E26 to E39, and T6 Circle ballast adapters. There are also cluster sockets available, so the adapter can hold two bulbs then affix to a single socket.