The spiral “Edison” type was designed and patented by Thomas Edison in 1909. The number next to the ‘E’ is the diameter of the spiral in millimeters.
The most used are:
The designation “MR” means “Multifaceted Reflector”. The number next to it is the diameter of the lamp in eighth-of-an-inch increments (1 inch equals 2.5 centimeters). For example: an MR16 is a multifaceted reflector with a diameter of 16/8 = 2 inches = 5 centimeters.
In the same way MR11 of diameter 11/8 = 1.37 inches = 3.5 centimeters.
These reflectors can have different connectors. Generally, pin-type connectors are for reflectors that operate at 12 volts with a transformer. There is also a connector called GU10 which is a “bayonet” type. It was designed to operate in 110-220V, therefore avoiding elecric shock or bulb explosion upon use of a 12v reflector in one of 120/220v.
The letters “PAR” mean Parabolic Aluminized Reflector. This reflector is common for gardens and parking lots and can be found in diverse colors. The cone shape is coated internally of a “mirror” type material that directs light forward in an enclosed cone. The number next to it is the diameter of the reflector in eighth-of-an-inch increments, therefore the PAR38 is a reflector of 38/8 = 4.75 inches = 12.1 centimeters of diameter