Snort is a lightweight network intrusion detection system, capable of performing real-time traffic analysis and packet logging on IP networks. Snort uses a flexible rules language to describe traffic that it should collect or pass, as well as a detection engine that utilizes a modular plugin architecture. Snort has a real-time alerting capability as well, incorporating alerting mechanisms for syslog, a user specified file, a UNIX socket, or WinPopup messages to Windows clients using Samba's smbclient. Snort has three primary uses. Setting up is simple, once unpacked read through the documentation, that is where you will find information on installing and using Snort.
Rules are what Snort looks for, like virus definition files it defines what to watch for.
By looking at the Snort website and reading the Current Snort Rule file you will see the flexibility of the definitions. If you want to watch for something specific you may create your own snort rule file and Snort will monitor it for you. Snort is a open source project and remains free to the user. Because unix based development has updates and changes often the link below goes directly to their download area. There you will download either the source or the RPM, and compile or install.