The hot shoe is shaped somewhat like an inverted, squared-off "U" of metal. The matching adapter on the bottom of the flash unit slides in from the back of the camera and is sometimes secured by a clamping screw on the flash. In the center of the "U" is a metal contact point. This is used for standard, brand-independent flash synchronization. Normally the metal of the shoe and the metal of the contact are electrically isolated from each other. To fire the flash, these two pieces are shorted together. The flash unit sets up a circuit between shoe and contact—when it is completed by the camera, the flash fires.
In addition to the central contact point, many cameras have additional metal contacts within the "U" of the hot shoe. These are proprietary connectors that allow for more communication between the camera and a "dedicated flash". A dedicated flash can communicate information about its power rating to the camera, set camera settings automatically, transmit color temperature data about the emitted light, and can be commanded to light a focus-assist light or fire a lower-powered pre-flash for focus-assist, metering assist or red-eye effect reduction.
Internal camera circuit shorts center contact and shoe mount, thus triggering the flash. The trigger voltage (for a flash) between the center contact and the shoe have varied over the years, between manufacturers, and even in the same manufacturer. When the contacts with a shutter were mechanical contacts, the actual voltage did not matter too much as long as it did not cause arcing, but now with electronic triggering, it can cause problems.
The ISO 10330 specification allows for a trigger voltage of 24 volts, some manufacturers, particularly Canon, ask for no more than 6 volts. Some older flashes may have a high voltage, sometimes in the hundreds of volts.
It is possible to connect a high voltage triggering flash with a camera which can only tolerate 6 volts, through the use of an adaptor which isolates the two units. Also many radio triggers, e.g. PocketWizard, while giving a low voltage to the camera, can handle 200V from the flash port  thus isolating the camera from the flash's high voltage trigger.
History and use 
Before the 1970s, many cameras had an "accessory shoe" or "cold shoe", intended to hold flashes that connected electronically via an outboard "PC cable" (not meaning a computer: the term goes back to the synchronization method of the "Prontor/Compur" shutters of the 1930s), or other accessories such as external light meters, special viewfinders, or rangefinders. These earlier accessory shoes were mostly the same U shape, and thus provided the template for the introduction of the hot shoe.
Since 1988, Minolta switched to use a 4-pin proprietary slide-on auto-lock "iISO" connector. A compatible 7-pin variant, which allows battery-less accessories to be powered off the camera's battery existed as well, but was not widely used. Konica Minolta and Sony Alpha digital SLR cameras are based on Minolta designs and therefore used the same connector, officially named Auto-lock Accessory Shoe, as well up to 2012. Since the electrical protocol remained mostly compatible, TTL and non-TTL adapters exist to adapt ISO-based flashes to iISO hotshoes and vice versa.
Sony also used a variety of other proprietary hotshoes for other digital cameras and camcorders, including the ISO-based 6-pin Cyber-shot hotshoe, the 16-pin Active Interface Shoe (AIS) and the ISO-based 16-pin Intelligent Accessory Shoe (IAS). Some of their NEX cameras used a proprietary Smart Accessory Terminal (versions 1 and 2). In September 2012, Sony announced a new ISO-based 21+3 pin Multi Interface Shoe for use with their future digital cameras and camcorders of the Alpha, NEX, Handycam, NXCAM and Cyber-shot series. This quick-lock hotshoe is mechanically and electrically compatible with a standard 2-pin ISO-518 hotshoe, but electrically compatible with the previous Auto-lock Accessory Shoe with extensions, so that passive adapters ADP-AMA and ADP-MAA allow to use digital-ready iISO flashes on new cameras and some new Multi Interface Shoe equipment on older cameras, while providing compatibility with standard ISO-based equipment as well.
Canon uses a non-ISO-based 13+1 pin hot shoe, named Mini Advanced Shoe on some of its camcorders.
Modern Cold Shoes 
There is still a use for "cold shoes" i.e., a shoe mount without the "hot" connection. These are often used with off-camera flash, having one or more flash units mounted on stands and connected to the camera by cable or triggered by wireless means.
Non flash items on the Hot Shoe 
Manufacturers in the 21st century still make non-flash items which mount on the hot shoe of their cameras. For instance, for the Olympus XZ-1, one can buy items such as a stereo microphone or three different models of electronic viewfinder. Another example is the FotoSpot Geo-Tagging GPS's which utilize the hotshoe for mounting to the camera.
- "ISO 518:2006 - Photography - Camera accessory shoes, with and without electrical contacts, for photoflash lamps and electronic photoflash units - Specification". International Organization for Standardization. 12 May 2006. Retrieved 2 August 2011.
- "ISO 518:1977 - Photography -- Camera accessory shoes, with and without electrical contacts, for photoflash lamps and electronic photoflash units". International Organization for Standardization. 1977. Retrieved 2 August 2011.
- "Strobist: Don't fry your camera".
- "List of strobes and voltages".
- "Pocket Wizard owner's manual for PW II Plus, page 14".
- "Strobist looks at the Frio cold shoe".
- "Olympus's site for XZ-1 accessories".