Accessory Shoe & Hot Shoe
Common up until the 1970s, an accessory shoe, or cold shoe, is a square panel, generally found on the top of the camera, to which accessories, such as the flash, light meter, or special viewfinder, can be attached. There is no electronic connection between the accessory and the camera; a flash can be mounted to the camera and connected electronically via a cable.
A hot shoe looks the same as an accessory shoe but has a metal contact point in the center used for flash synchronization. When a flash is attached it completes a circuit that fires the flash when the shutter is released. There is no need for a cable in this case. More complex contacts allow for more advanced communication between the camera and the accessory.
Flash Unit Connector Socket (PC Socket)
A PC Socket (or synchroflash socket) is often found on the body of pre-hot shoe cameras. It was introduced in the 1950s by two German leaf shutter manufacturers (Prontur-Compur) as a standardized way to synchronize their shutters with a flash via a sync cable. It is still featured on some modern cameras and is mainly used to fire a studio flash that isn’t mounted directly to the camera. Some cameras will feature multiple PC sockets, each labeled for a specific type of flash.