A packet sniffer is a wire-tap devices that plugs into computer networks and eavesdrops on the network traffic. Like a telephone wiretap allows the FBI to listen in on other people's conversations, a "sniffing" program lets someone listen in on computer conversations.
Sniffing programs have been around for a long time in two forms.
Commercial packet sniffers are used to help maintain networks.
Underground packet sniffers are used to break into computers.