In a former life I was a network guy. In that time I saw the rise switches instead of hubs for networks. For those of you who were not networkers here is the definition:

Hubbed Network
The hub is a device that acts as a repeater out to the other connected computers. If I am trying to talk to Joe's computer every other computer receives that information. The other computers investigate the packet and drop it as it is not destined for them. Joe's computer accepts the packet as he says it is for him.

Switched Network
The switch is a smarter device that knows which computers are connected to it and which port on the switch they are plugged into. When a packet is sent the switch looks up the destination and sends the proper destination port.

For those of you that have run a packet sniffer on a hubbed network you will see all the traffic from all computers. Some of these things include email messages and passwords (yes POP3 sends your password in plain text over the wire), instant messaging conversations, oh and maybe some actual work documents being passed around.

When you run a sniffer on a switched network all you see is the traffic from your computer to the destination. This is because the switch is being the traffic cop and redirecting everything properly.

Switches were invented to be traffic cops, not security cops. It is actually fairly trivial to trick the switch into thinking that either you are the intended destination (via ARP poisoning in the initial traffic notification) or to convince the router that one computer is hooked up to the switch twice (via MAC address duplication). It is also possible to flood the switch until it crashes and fails open into a hub configuration until the flood stops.

If you want more technical details sans.org has a good article here:
http://www.sans.org/resources/idfaq/switched_network.php