From LQWiki

A hub (once called a "concentrator, or an "aggregator") is a hardware device that allows 2 or more network clients to share a single collision domain.

Network hub

Hubs are considered out of date for modern network installations. This is because they create a large collision domain. You can get much improved performance by using a network switch, although for very small networks, eg, up to 30 machines, a hub could be sufficient for your needs.

USB hub

A USB hub allows a single usb port on a Computer to have lots of USB devices connected to it. Bear in mind that when you use a hub you limit everthing on the hub to the speed of the hub. Quite a few devices, (particulary those using USB 1 or those needing lots of bandwidth, like external hard disks) will refuse to work when connected through a hub. Ideal uses for a hub include modems, keyboards, and mice.