Such a hub is an unintelligent network distributor box, which is used for Ethernet. Typically the hub can connect 4 or 8 pc's together by having a cable from each net card to the hub. Each plug-in is called a port.
Hubs are available in 10 or 100 Mbit editions. This number indicates the total bandwidth in the box. If 8 pcs exchange data simultaneously through the same 100-bit hub, there will in reality be a maximum of 10 mbit available to each connection.
The hub is unintelligent (contrary to the switch-box). The hub receives signals from a network card, which want to communicate with another network card. This signal is amplified in the hub, and then sent to all the ports. Then it is up to the receiver to collect the data sent. This causes a lot of "noise" in a hub, since all data have to be sent to all ports to find the proper receiver. You can compare it shooting a shotgun without a choke.
Please see the switch, which is a much more intelligent and effective box.