USB

USB (Universal Serial Bus) is currently the most commonly used connector to connect various things to computers, replacing what used to be a great profusion of serial, parallel, and other connectors used to attach printers, keyboards, mice, and other peripherals and devices. It is also widely used for charging batteries of mobile devices, replacing another profusion of proprietary connectors that used to be used (and is still sometimes used, for instance with Apple replacing the proprietary iPhone connnector with another proprietary iPhone connector not compatible with the old one).

USB may get even more useful in the future, as a revision of the standard will make it possible to use higher wattages of power through USB, leading to it being a possible replacement for old-fashioned electric outlets for many purposes (getting around the confusing variety of outlet types in different countries), and increasing the use of direct current instead of the alternating current which has dominated for over a century. When the current USBs are used for charging, they can often be plugged into AC adapters that plug into electric outlets, accomplishing the task of charging without hooking the device up to a computer.

The increasing number of things that connect to USB ports has resulted in new models of computers generally having increasing numbers of spots to plug them in, and often other devices such as monitors will have a few more. People still keep running out of them, so expansion plug units are popular as well.

A number of versions of the USB standard have been released, with 3.1 being the latest. A new USB standard will reportedly support reversible plugs (ending one area of confusion in the current standard where it's not easy to see which way the things need to be inserted but they won't fit the other way), but uses a plug that's incompatible with the old ones, thus leading to a new round of devices whose plugs won't fit into your computer. Even the old USBs aren't quite "universal"; while the end that plugs into a computer is pretty standard, the other end (plugging into a device being hooked up to the computer) will often be of one of several "micro-USB" plugs (more compact, designed for portable mobile devices) or some other plug (like the proprietary Apple ones).

Specs

 * USB 1.0 spec
 * USB 3.0 spec

Other links

 * Wikipedia article
 * Edison's Revenge
 * USB plug goes both ways
 * Power over USB: when charging a computer means connecting to untrusted data-sources
 * Public ‘nowhere near ready’ for reversible USB (satire)
 * BadUSB - On Accessories that Turn Evil by Karsten Nohl + Jakob Lell
 * How many kinds of USB-C™ to USB-C™ cables are there?
 * Why You Should Never Borrow Someone Else's Charging Cable