Switches are hardware devices for networks that prepares packets to be sent to a router(or possibly another kind of network device). In doing so, they separate the data into ethernet segments. For many years it had been the belief that this protected systems on separate ethernet segments from having their transmissions sniffed. While physical network architecture plays a large part on how much traffic can be intercepted, it is generally the case that if a computer is connected to the switch, and forges ARP(Address Resolution Protocol) replies of the network's gateway it can sniff traffic across ethernet segments, thus dissolving the age-old myth.
For proof of concept, look at dugsong's utility arpredirect, at