Why one only tells you about the attack, and the other can stop it mid-flight.
An Intrusion Detection System (IDS) and an Intrusion Prevention System (IPS) both watch traffic for signs of malicious activity, and the acronyms get treated as almost interchangeable — as if an IPS were just "an IDS with auto-block turned on." It isn't. The real difference is where each device physically sits relative to the traffic, and that placement decision is what determines whether the system can ever do anything besides tell you what already happened.