A USB hub is a small device that contains a number of USB ports where different USB devices can be plugged in. The USB hub is usually connected to an unused USB port on the computer using an extension cable. Upon plugging in, the USB hub serves as multiple input for different USB devices, which is similar to the purpose of an multi socket electrical extension cord.
A USB hub is seen as an answer to the early limitations of computers. When USB devices where first introduced to the market, most new computers only had one or two USB ports. But because of the explosive popularity of USB devices, the limited number of ports are not enough to serve the growing number of devices that need to be plugged in. The solution is to create a USB hub that basically multiplies the number of USB ports in a computer without having to drastically upgrade the computer hardware.
There are self powered USB hubs that serve perfectly as a connection for digital cameras, card readers, mice, MP3 players, keyboards and other handheld USB devices. For components and peripherals that need a little more power, there are AC-powered USB hubs, that come with its own AC adapter. These powered USB hubs are perfect for peripherals like portable hard drives, printers, and scanners. The rule of thumb when purchasing a USB hub is to look at your needs and requirements. Consider the type of USB devices that you will need to plug into the USB hub and from there decide on whether to get a self-powered hub or an AC-powered USB hub.