A USB hub is a device that expands a single USB port into several so that there are more ports available to connect devices to a host system.
 
USB hubs are often built into equipment such as computers, keyboards, monitors, or printers. When such a device has many USB ports they all usually stem from one or two internal USB hubs rather than each port having independent USB circuitry.
 
Physically separate USB hubs come in a wide variety of form factors: from external boxes (looking similar to a network hub) connectible with a long cable, to small designs that can be directly plugged into a USB port (see the 'compact design' picture). In the middle case, there are "short cable" hubs which typically use an integral 6 inch cable to slightly distance a small hub away from physical port congestion and of course increase the number of available ports.
 
Laptop computers may be equipped with many USB ports, but an external USB hub can consolidate several everyday devices (like a mouse and a printer) into a single hub to enable one-step attachment and removal of all the devices.