USB (Universal Serial Bus) is an industry-standard type of plug-in connection that is used to connect devices, including phones, to desktop or laptop computers. It is also used for charging.
In a phone, USB is useful for quickly transferring files to and from phones, or for synchronizing address book and calendar information with a computer application such as Outlook.
In addition to carrying data, USB carries power, and can therefore be used to charge phones. (Some older phones do not support charging via USB, even though they have standard mini-USB connectors.)
There are standard USB connectors (mini and micro) that are found in some phones. However, some phones have a proprietary connector that requires an adapter cable to connect to a PC via USB.
USB has several versions. USB 2.0 is newer and potentially up to 40 times faster than USB 1.1. To benefit from the faster speed, the device must support "Hi-Speed" mode. "Full speed" is a misleading term that actually means slow speed (the same as USB 1.1.)
The newest version is 3.0. The fastest speed is "Super Speed", which is 10 times faster than Hi-Speed.
A computer may require a special driver or other software to communicate with a phone over USB in a specific way. For example, not all computers support MTP and PTP modes for transferring photos and other media.
Alternatively, many newer phones support USB "mass storage" mode, which does not require any special drivers or software with modern computers, although this only supports file transfer.
See: Mass Storage mode
Some phones support USB-Host (also called USB-OTG), which allows USB accessories to be connected directly to the phone, with no computer involved.
See: USB OTG
Some phones support PictBridge, which specifically allows a phone to connect directly to a printer via USB: