Hubs:

A hub joins multiple computers (or other network devices) together to form a single network segment. Hubs are actually multiport repeaters. In many cases, the difference between the two devices is the number of ports that each provides. While a typical repeater has just two ports, a hub generally has from four to twenty-four ports.
Using a hub changes the network topology from a linear bus, where each device plugs directly into the wire, to a star. With hubs, data arriving over the cables to a hub port is electrically repeated on all the other ports connected to the same network segment, except for the port on which the data was sent.
Hubs come in three basic types:
• Passive – A passive hub serves as a physical connection point only. It does not manipulate or view the traffic that crosses it. It does not boost or clean the signal. A passive hub is used only to share the physical media. As such, the passive hub does not need electrical power.
• Active – An active hub must be plugged into an electrical outlet because it needs power to amplify the incoming signal before passing it out to the other ports.
• Intelligent – Intelligent hubs are sometimes called smart hubs. These devices basically function as active hubs, but also include a microprocessor chip and diagnostic capabilities. Intelligent hubs are more expensive than active hubs, but are useful in troubleshooting situations.
Devices attached to a hub receive all traffic traveling through the hub. The more devices there are attached to the hub, the more likely there will be collisions. A collision occurs when two or more workstations send data over the network wire at the same time. All data is corrupted when that occurs. Every device connected to the same network segment is said to be a member of a collision domain.
Sometimes hubs are called concentrators, because hubs serve as a central connection point for an Ethernet LAN.