Most retrofits have the appearance of a conventional lamp and are used as direct replacements for the existing one. I.e. they have a screw or bayonet cap base. In downlights and spotlights, it s common to have a 50mm dia reflector lamp. The mains voltage ones are usually called GU10 which refers to the flattened pins on the base. However, some are available for 12V supply fed from a transformer, e.g. Direct replacements for 50mm dichroic LV downlights. These will have thinner pins and are often called MR16 or GU5.3 lamps.