USB is the Universal Serial Bus.
USB replaces the old serial ports used on computers from back when Noah used a teletype to control the Satnav device on his ark. The technical details of USB are available for hardware developers at http://www.usb.org.
USB can also replace the keyboard and mouse ports on your computer. USB was first available for a Microsoft operating system in NT but Microsoft suppressed USB in NT to force everyone on to Windows 2000. Windows 2000 let you replace both the mouse and keyboard with USB devices but would often fail if you tried to keep your reliable old keyboard and add only a USB mouse. Microsoft finally released a working USB in Windows XP.
Some distributions of Linux had a working USB system before XP became stable with the release of Service Pack 2 for XP. Some distributions of Linux were 64 bit before a stable version of Windows XP 64 was released. Some distributions of Linux have easy to use RAID, a feature that Microsoft artificially restricts to their expensive server products. USB arrived at a time when Linux was overtaking Windows. Today USB is no longer a deciding factor for choosing an operating system.
USB 3.0 is the latest version of USB and is a deciding factor for choosing hardware. Some computers have, at July 2010, a small number of USB connectors upgraded from USB 2.0 to USB 3.0.
Modern internal disks are connected using SATA. A USB 2.0 connection is slower than the fastest SATA disks. Some computers added an eSATA connection to let you connect an external SATA drive direct. USB 3.0 is faster than the current SATA II and USB 3.0 lets you use a cable many times longer than eSATA. There is a new SATA III on the way that is slightly faster than USB 3.0 but not enough to put up with the very short cables.
A SATA cable is limited to less than a metre (3 feet). External eSATA cables are typically 0.5 m (1.5 feet). USB 3.0 cables are available up to 3 m (10 feet). USB 3.0 is long enough to have your computer on a shelf next to your desk and place the USB drive on your desk.
You can run one USB cable from your computer to your desk, place a USB hub on the end of the cable, then plug several USB devices into the hub, reducing the number of cables sprouting out of your computer.
USB cables carry enough power for keyboards and mice but not disks. A single USB 2.0 cable can deliver enough power to run a hub with a mouse and keyboard plugged in or a single DVD drive but not both. If you plug several devices into a USB hub, you need a powered hub. Powered hubs have their own power supply and can a wide range of devices. Disks use just a little too much power for USB 2.0 and need their own power supply.
USB 3.0 is designed to carry more power, enough for a modern high speed solid state disk and some low power mechanical disks. You could use one USB 3.0 cable for one disk or two optical drives.
Hubs are great when you use one device at a time. USB 2.0 is fast enough to use a keyboard, a mouse, and listen to music from an external USB device. USB 2.0 is not fast enough to copy data from one USB disk to another using one cable and a hub. USB 3.0 is fast enough to copy data from a DVD through a hub to your computer while copying data out to an external DVD drive or disk through the same hub.
You will only get USB 3.0 speed if every device, cable, and connection is USB 3.0.
A full backup of a medium size disk to an external USB 2.0 disk can take eight hours, something you might do overnight. USB 3.0 could reduce that to less than an hour, something you could do over lunch. You are more likely to run frequent backups when using USB 3.0 speed.
USB does away with a lot of existing cables. USB 3.0 will replace eSATA and make external disks as fast as internal disks.