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. We are sure to see a Mac OS X install package in the near future for this application for now you have to be a little unix savvy. One of the great things about Snort is it is BSD compatible so Mac OS X users may use this free program to run network intrusion tests. If your interested in security this is a must for Mac OS X users.