Chapter 7 Firewall for FortiOS 5.0 : Firewall objects : Security profiles : Intrusion Protection (IPS)
  
Intrusion Protection (IPS)
Intrusion Prevention System is almost self explanatory. In the same way that there is malware out on the Internet that the network needs to be protected from there are also people out there that take a more targeted approach to malicious cyber activity. No operating system is perfect and new vulnerabilities are being discovered all of the time. An intrusion prevention system is designed to look for activity or behavior that is consistent with attacks against your network. When attack like behavior is detected it can either be dropped or just monitored depending on the approach that you would like to take.
As new vulnerabilities are discovered they can be added to the IPS database so that the protection is current.