What is USB?

USB, short for Universal Serial Bus, is a specification to establish communication between devices and a host controller (usually personal computers). A single USB port can be used to connect up to 127 peripheral devices, such as keyboards, speakers, mice and scanners. And it's hot-swappable, which means that a device can be connected or disconnected while the computer is running.

The following are some major Versions of USB

USB 1.0\1.1: USB 1.0 was released in 1996 and it supports data transfer rates of 12 Mbps. USB 1.1 was released in 1998. It fixed problems identified in 1.0, mostly relating to hubs.
USB 2.0: also called Hi-Speed USB, an external bus that supports data rates up to 480Mbps. USB 2.0 is an extension of USB 1.1. USB 2.0 is fully compatible with USB 1.1/1.0 and uses the same cables and connectors. Currently, computers are built with USB 2.0 ports
USB 3.0: USB 3.0 is also called SuperSpeed USB. It can offer transfer rates of 4.8 gigabits per second and compatible with USB 1.1/1.0/2.0. And the first computer with USB 3.0 has come out.

See: External Hard Drive, SD Card, CF Card, iPod, iPhone