There are a large number of possible fittings for the various lights. Here are the most common:
Screw fittings: The best known are the small (E14) and the large (E27). We also know the Goliath fitting (E40).
Plug fittings: This means that the bulb has one or more feet, allowing it to be inserted into the socket in one movement. It usually clamps the foot(s) automatically, but sometimes it needs to be screwed. Plug fittings are available from very small to quite large.
TL fittings: Straight fluorescent tubes have their own fitting systems.
Bayonet fittings: These fittings are pushed into the lamp so that the pins on the side can get caught behind an opening.