A Sniffer, which is can also be referred to as a Network Analyzer, is a piece of software that analyze network traffic, decode it, and give it back packet information so that a network administrator can use it to help diagnose problems on the network. But because these tools can be so powerful, they can also help leverage those of the black hat world by allowing them to pull plain text information off the network as well (usernames, passwords, unencrypted emails, instant message chat, etc).