Hubs

Hubs are devices that connect other devices together over a network. They are inherently a "broadcast" device, in that all devices see all traffic that passes through the hub, even if they are neither the sender nor receiver of the traffic. This makes hubs both insecure (since everyone sees the traffic, and can sniff the traffic), and perform poorly (due to the increased likelyhood of collisions).

A switch should be used instead of a hub in almost all cases. One exception to this is when diagnosing connections, and you do need to eavesdrop on the traffic (perhaps when testing firewall rules). A hub can be (more or less) transparently used to "fork" the traffic to another device, such as a laptop running tcpdump or other diagnostic software.