Hub
An Ethernet hub is a multi-port repeating device that transmits data it receives on one port to all its other ports. It is often used as the centre of a cable network, and connects to each device in the network over a separate cable.
The simplest and most common form of hub will just have a number of 10MBPS 10BaseT or 10/100 ports. Port numbers from four to 24 are most common, although some chassis devices can have hundreds.
Different hubs can communicate at different speeds, and some have Autonegotiating ports.
There is a special type of hub called a switch that selectively repeats data as a way of reducing network traffic and improving throughput.
Hubs can be connected to each other to create larger
networks, but there are rules about how this can be done - 
for
more details.
See also: Ethernet Switching Half / Full Duplex