Definition: Packet sniffing is the act of capturing packets of data flowing across a computer network. The software or device used to do this is called a packet sniffer. Packet sniffing is to computer networks what wire tapping is to a telephone network.
Packet sniffing has legitimate uses to monitor network performance or troubleshoot problems with network communications. However, it is also widely used by hackers and crackers to gather information illegally about networks they intend to break into. Using a packet sniffer it is possible to capture data like passwords, IP addresses, protocols being used on the network and other information that will help the attacker infiltrate the network.