Snort is an open source network intrusion detection and prevention system.
It is capable of performing real-time traffic analysis, alerting, blocking
and packet logging on IP networks. It utilizes a combination of protocol
analysis and pattern matching in order to detect a anomalies, misuse and
attacks.
Snort uses a flexible rules language to describe activity that can be
considered malicious or anomalous as well as an analysis engine that
incorporates a modular plugin architecture. Snort is capable of detecting
and responding in real-time, sending alerts, performing session sniping,
logging packets, or dropping sessions/packets when deployed in-line.

Snort has three primary functional modes.  It can be used as a packet
sniffer like tcpdump(1), a packet logger (useful for network traffic
debugging, etc), or as a full blown network intrusion detection and
prevention system.