A hub (sometimes called a repeater) is a layer 2 networking device, similar to a switch
, but not as advanced. Instead of sending a datagram to a specific ethernet address, the hub will simply forward the datagram to every port, except for the one it came from. They are common in small networks, but the massive overhead caused by all the datagrams being sent to every device on the network makes them extremely impractical in large networks. They are becoming less common as time passes on. However, they are immune to looped cabling in the network, but more and more switches are equipped with the Spanning Tree Protocol, which prevents against issues with looped connections. Wiring the network correctly also helps with this.
Hubs are used to expand a network. If you need to add devices, and there aren't enough ports available, time to get a hub or a switch.