USB is an international standard of technology for connecting peripherals to computers and other devices that support it. An interface where a USB device can be connected is called a port. A USB hub is a special device which allows adding more USB ports. The additional ports share one USB connection to the computer; for this reason, each of them cannot carry as much data as a direct USB connection can.
There are different versions of the USB standard; main difference between the version is the maximum speed that is supported. The USB standard also defines different types of USB plugs.
A USB hub is a normal USB device, which was built for a given version of the standard. Certain models may support different versions of USB, while others may not. As an example: USB 3.0 can can transfer payload data at up to 4 GBits per second.USB 2.0 is limited to a transfer rate of 280 MBits per second. Connecting an USB 3.0 device to the computer using an USB 2.0 hub will limit the transfer rate to that of USB 2.0.