What is the main difference between IDS and IPS?

A.
In-line versus distributed

B.
Preventative versus detective

C.
Defense-in-depth versus a layered approach

D.
Stateful versus non-stateful

Explanation:
Traditional IDS only detects that something bad may be taking place and send an alert. The goal of IPS is to detect this activity and not allow the traffic into gain access to the target in the first place. So IPS is a preventive and proactive technology, where IDS is a detective and after-the-fact technology. Although some IPS products are in-line, this is not the main difference between these approaches.