Just about any computer that you buy today comes with one or more Universal Serial Bus connectors on the back. These USB connectors let you attach everything from mice to printers to your computer quickly and easily. The operating system supports USB as well, so the installation of the device drivers is quick and easy, too. Compared to other ways of connecting devices to your computer (including parallel ports, serial ports and special cards that you install inside the computer's case), USB devices are incredibly simple!
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Connecting a USB device to a computer is simple -- you find the USB connector on the back of your machine and plug the USB connector into it.

A typical USB connector, called an "A" connection. If it is a new device, the operating system auto-detects it and asks for the driver disk. If the device has already been installed, the computer activates it and starts talking to it. USB devices can be connected and disconnected at any time.
Many USB devices come with their own built-in cable, and the cable has an "A" connection on it. If not, then the device has a socket on it that accepts a USB "B" connector. A typical "B" connectionThe USB standard uses "A" and "B" connectors to avoid confusion:

* "A" connectors head "upstream" toward the computer.
* "B" connectors head "downstream" and connect to individual devices.
By using different connectors on the upstream and downstream end, it is impossible to ever get confused -- if you connect any USB cable's "B" connector into a device, you know that it will work. Similarly, you can plug any "A" connector into any "A" socket and know that it will work.

Running Out of Ports?
Most computers that you buy today come with one or two USB sockets. With so many USB devices on the market today, you easily run out of sockets very quickly. For example, on the computer that I am typing on right now, I have a USB printer, a USB scanner, a USB Webcam and a USB network connection. My computer has only one USB connector on it, so the obvious question is, "How do you hook up all the devices?"
The easy solution to the problem is to buy an inexpensive USB hub. The USB standard supports up to 127 devices, and USB hubs are a part of the standard.

A hub typically has four new ports, but may have many more. You plug the hub into your computer, and then plug your devices (or other hubs) into the hub. By chaining hubs together, you can build up dozens of available USB ports on a single computer.

Information resource "How Things Work"