Most computers that you buy today come with at least one or two USB sockets. But with so many USB devices on the market, you easily run out of sockets very quickly. For example, you could have a keyboard, mouse, printer, microphone and webcam all running on USB technology, 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.
Hubs can be powered or unpowered. As you'll see on the next page, the USB standard allows for devices to draw their power from their USB connection. A high-power device like a printer or scanner will have its own power supply, but low-power devices like mice and digital cameras get their power from the bus in order to simplify them. The power (up to 500 milliamps at 5 volts for USB 2.0 and 900 milliamps for USB 3.0) comes from the computer. If you have lots of self-powered devices (like printers and scanners), then your hub doesn't need to be powered -- none of the devices connecting to the hub needs additional power, so the computer can handle it. If you have lots of unpowered devices like mice and cameras, you probably need a powered hub. The hub has its own transformer and it supplies power to the bus so that the devices don't overload the computer's supply.