Hubs -
A hub is a device that is used to extend an Ethernet wire to allow more devices to communicate with each other. When using a hub, the network topology changes from a linear bus, where each device plugs directly into the wire, to a star. Data arriving over the cables to a hub port is electrically repeated on all the other ports that are connected to the same Ethernet LAN, except for the port on which the data was received. Sometimes hubs are called concentrators, because they serve as a central connection point for an Ethernet LAN. Hubs are most commonly used in Ethernet 10BASE-T or 100BASE-T networks, although there are other network architectures that use them.

Bridges and Switches
Bridges connect network segments. The basic functionality of the bridge resides in its ability to make intelligent decisions about whether to pass signals on to the next segment of a network. When a bridge sees a frame (data being sent from one computer to another) on the network, it looks at the destination address and compares it to the forwarding table to determine whether to filter, flood (sent to everyone), or copy the frame onto another segment.

A switch is sometimes described as a multiport bridge. While a typical bridge may have just two ports (linking two network segments), the switch has several ports depending on how many network segments are to be linked. A switch is a more sophisticated device than a bridge, although the basic function of the switch is deceptively simple. It is to choose a port to forward data to its destination. Ethernet switches are becoming popular connectivity solutions because, like bridges, they increase network performance (speed and bandwidth).

Routers
Routers are the most sophisticated internetworking devices discussed so far. They are slower than bridges and switches, but make "smart" decisions on how to route (or send) packets received on one port to a network on another port. Each router interface is connected to a different network segment. Routers can be computers with special network software installed on them or they can be other devices built by network equipment manufacturers. Routers contain tables of network addresses along with optimal destination routes to other networks.