IDS (Intrusion Detection System)'s are primarily programs tasked with detecting and logging malicious web traffic.
IPS (Intrusion Prevention System) are tasked with stopping the aforementioned malicious traffic.
IDPS (Intrusion Detection and Prevention Systems) are the combination of the two, and the best way to put them to use.

The IDS allows you to view network connections while the IPS allows intervention and control over the users' actions. It is recommended that clients take this service during or right after the system's first put together and used. The service is usually performed on scene.

The most important reason to get this service is to get previously known attacks and other sorts of unauthorized access blocked and reported before ever reaching IT resources.