USB = "Universal Serial Bus" - This is a new way to connect devices like scanners, digital cameras, external hard disks, keyboards, mice, and so on. With the old-style Serial Ports (for mice and some digital cameras) and Parallel Ports (for printers and scanners), you have to turn OFF the computer to connect the device or risk damaging both the computer and the device. And when using the device, you must usually turn the device ON first before you turn ON the computer, or the computer won't be able to find the device.
With USB, you can "hot-plug" the device in while the computer and the device are turned ON. The computer will then load the software it needs for the device automatically and link up with the device. The next time you start the computer, the computer should automatically find the device during the start-up process and not have to load the software again. [In actual fact, you sometimes need to unplug the device and plug it back in to get the computer to recognize it. (They are still working the bugs out of USB.)]
With USB, you can connect many, many devices up to the same port by using a "hub" to split off the port. A hub is a small box that has many USB connectors on it and one USB cable that plugs into the back (or front) of newer computers. All of the devices should theoretically work harmoniously together, but, in fact, can sometimes conflict with each other. You may then need to change the way they are all connected together, or download a new "patch" or "fix" from the manufacturer's web site to correct the problem.