Direct current (DC) is a form of electricity. Since most wall sockets provide alternating current (AC) power, a device will need to convert the supplied electricity before it enters the device. A large, boxy transformer will convert the power, which can then leave the cord through the DC connector. There are several different designs for these connectors. The most common style is a simple round plug, but several specialized styles exist as well.
Many forms of electronic devices only work with DC power. If the device were to have AC power supplied instead, it would simply not function, or it may have a catastrophic failure. On the other hand, AC is a much better way of transferring electricity over large distances through power distribution systems and household wiring. In order to deal with this discrepancy, many devices use a transformer on the power system that converts AC to DC.
A DC connector is part of this system whenever the cord is detachable from the device. Detachable cords consist of four parts, the first being the cord itself. The AC plug connects to the wall and uses a standard two- or three-prong connection. If the cord has a large box, either as part of the AC plug or on the cord somewhere, that is a transformer than coverts AC to DC power. Some systems have the transformer inside the device itself. Lastly, the DC connector is opposite the AC plug and it plugs directly into the device.
The round metallic plug type of DC connector typically look like a hollow cylinder. Some round connectors, often called post connectors, are simply a solid piece of metal. This kind of DC connector is common is audio equipment such as headphones. There are no official specifications for these types of connectors, so there are a wide range of proprietary designs.
While the round DC connector style is the most common, there are several others. The miniature Deutsches Institut für Normung (mini-DIN) plug is slightly larger around than a pencil and has between three and ten pins inside a protective metal jacket. The mini-DIN DC connector is common in a wide number of places, particularly computers and audio/video equipment. Separate video (S-video) connectors are a common example of this plug style.
Another common non-standard DC connector is the Molex connector. These connections are the standard four-pin leads found on nearly every computer power supply. A Molex connector is the standard method of supplying power to computer drives and internal peripherals.