Ethernet hub

An Ethernet hub or concentrator is a device for connecting multiple twisted pair or fiber optic Ethernet devices together, making them act as a single segment. Hubs work at the physical layer (layer 1) of the OSI model. The device is thus a form of multiport repeater. Ethernet hubs are also responsible for forwarding a jam signal to all ports if it detects a collision.

Hubs also often come with a BNC and/or AUI connector to allow connection to legacy 10BASE2 or 10BASE5 network segments. The availability of low-priced Ethernet switches has largely rendered hubs obsolete but they are still seen in older installations and more specialist applications.

Usefulness:

Historically, the main reason for purchasing hubs rather than switches was price. This has largely been eliminated by reductions in the price of switches, but hubs can still be useful in special circumstances:

• A protocol analyzer connected to a switch does not always receive all the desired packets since the switch separates the ports into different segments. Connecting the protocol analyzer to a hub allows it to see all the traffic on the segment. (Expensive switches can be configured to allow one port to listen in on traffic on another port. However, these cost much more than a hub.)
• Some computer clusters require each member computer to receive all of the traffic going to the cluster. A hub will do this naturally; using a switch requires implementing special tricks.
• When a switch is accessible for end users to make connections, for example, in a conference room, an inexperienced or careless user (or saboteur) can bring down the network by connecting two ports together, causing a loop. This can be prevented by using a hub, where a loop will break other users on the hub, but not the rest of the network. (It can also be prevented by buying switches that can detect and deal with loops, for example by implementing the Spanning Tree Protocol.)
• A cheap hub with a 10BASE2 port is probably the cheapest and easiest way to connect devices that only support 10BASE2 to a modern network (cheap switches don't tend to come with 10BASE2 ports). The same goes for linking in an old thicknet network segment using an AUI port on a hub (individual devices that were intended for thicknet can be linked to modern Ethernet by using an AUI-10BASE-T transceiver).

(From Wikipedia)