A lampholder, usually medium or candelabra screwbase, intended to be attached to a wire to form a string of lights (a festoon). The same item can often be mounted (with a bit of ingenuity, often involving hot glue) to create a marquee or to outline a portal or other 2D scenic piece.
One of many examples:
http://www.actionlighting.com/medium-base-leviton-167-socket-with-hook-216sk-l167skt-h/
One of many examples:
http://www.actionlighting.com/medium-base-leviton-167-socket-with-hook-216sk-l167skt-h/