USB is a standard for a wired connection between two electronic devices, including a mobile phone and a desktop computer. The connection is made by a cable that has a connector at either end. One end, the one that plugs into the computer, is the same across all USB cables while the one that plugs into the mobile device can be of various types such as miniUSB, microUSB or a proprietary connector.
USB version 1.1 provides maximum speeds of up to 1.5 MB/s while the current version 2.0 is about 40 times faster. The versions are backwards compatible and the speed is limited by the slower device. Transferring data may require drivers to be installed on the desktop computer but some phones offer "mass storage" mode which means they appear as thumb drives to the computer and no special drivers are needed.
In addition to their data transferring application, USB cables also carry an electric charge that can be used to power peripherals (such as USB mice or keyboards), and many mobile phones can be charged through their USB port.