The shell is the outside of a lamp socket. The shell is usually metallic and colored black or gold. It serves no functional purpose. Consequently, more elaborate lamps may not have a shell; instead, the socket may be built directly into the lamp fixture.
In addition to the main cylindrical part of the shell, which encases the rest of the socket, some lamp socket shells have a second piece, a cap which goes underneath the base of the socket. The cap usually has a hole for the power cord to pass through.
The outer shell wraps around a layer of insulation, which protects the shell (and any person touching the shell) from the heat and electric current that can be present in the inner socket. The insulation can be made of any material with good thermal and electrical insulation properties, such as rubber, mica, PVC and many other materials.
Socket Base & Current In
The power cord holds at least two wires, a hot wire carrying current into the lamp, and a neutral wire carrying current back out. The hot wire of the power cord connects to the base of the lamp socket at a terminal which usually consists of a screw. From there, the current flows up to a metal tab that sits at the bottom of the exact center of the socket interior. This tab is where the bottom tip of the light bulb will make its hot connection. The tab sticks up slightly, so the light bulb can press down on it and make a secure connection.
Socket Threading & Current Out
Light bulbs screw into lamp sockets, which requires both the bulbs and the sockets to have a threaded interface. The socket's threading is built inside the insulation sleeve. It is made of metal, and serves the double purpose of completing the neutral connection between the light bulb and the lamp. The electric current flows out of the bulb and into the socket threading. From there, it passes into another terminal screw. The neutral wire is wrapped around this screw, and carries the current back into the power cord.
Every lamp socket consists of a switch which, when shut, completes the circuit and powers the light bulb. The switch, which can be a turn switch or a pull switch, makes its connection with the circuit inside the lamp socket, safely out of sight. Some more elaborate switches have four different settings instead of just two, and can accommodate bulbs designed to put out three different levels of light.