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USB is by far the most popular method for connecting external hardware to a PC. USB has three somewhat confusingly-named data transfer speeds, and two versions available. USB 1 supports "Low Speed" and "Full Speed" (1.5 and 12 Mbit/s respectively), and USB 2 adds "Hi-Speed" (480 Mbit/s). USB 2 devices are usually backwards compatable if your PC only has USB 1 capabilities. There are two major physical connector types available, A and B. A is the familiar rectangular connector you will find on hardware and cables that connects to the PC. B type connectors are found on the peripherals themselves, if the cable is removable (such as on a digital camera). There are also Mini-B type connectors that are often used with digital cameras, which can sometimes be specific to the hardware manufacturer. The connectors are all keyed, so they can only be inserted into the devices in one orientation. USB devices can be plugged and unplugged from the PC at any time without damaging the device or the PC, though with some devices, the operating system needs to be told you are going to disconnect the device so that data transfer isn't cut unexpectedly.
There is a huge variety of USB peripherals available for the PC market. Keyboards and Mice were the first widely-adopted peripherals to use the USB standard. Digital cameras, USB "stick" memory, and external hard drives make good use of USB2's much faster data transfer rate, allowing you to transfer files almost as fast as to an internal hard disk. There are modems and network interface cards available in USB.
Most motherboards today come with USB as an integral part of the board, with at least two connectors available, with four, six, and even eight connectors becoming more popular. There are PCI expansion cards available that provide USB for motherboards that do not have it built in, or for those who need more USB connectors. There are also USB expansion hubs available that replicate USB ports if you need more connections.
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