Most retrofits have the appearance of a conventional lamp and are used as direct replacement for the existing one. i.e. they have a screw or bayonet cap base. With 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.