A sniffer is an application that can capture network packets. Sniffers are also known as network protocol analizers. While protocol analyzers are really network troubleshooting tools, they are also used by hackers for hacking network. If the network packets are not encrypted, the data within the network packet can be read using a sniffer. Sniffing refers to the process used by attackers to capture network traffic using a sniffer. Once the packet is captured using a sniffer, the contents of packets can be analyzed. Sniffers are used by hackers to capture sensitive network information, such as passwords, account information etc.
Many sniffers are available for free download. Leading packet sniffers are wireshark, Dsniff, Etherpeek, sniffit etc.