Network security consists of the provisions made in an underlying computer network infrastructure, policies adopted by the network administrator to protect the network and the network-accessible resources from unauthorized access and the effectiveness (or lack) of these measures combined together.
Network Sniffing
Network sniffing refers to a computer connected to a network through "promiscuous mode", listening to every bit of traffic that goes by on the network and captures all the data. Normally a computer's network connection ignores traffic that is not addressed to it, but with sniffing software the computer will pick up everything. This is an easy way to pick up clear text passwords. E-mail programs frequently use clear text passwords.
From : http://www.networkdictionary.com/security/n.php#13