Although IPS and IDS both examine traffic looking for attacks, there are critical differences. IPS and IDS both detect malicious or unwanted traffic. They both do so as completely and accurately as possible, at the speed of the network. But an IPS is an in-line device designed for automatic enforcement of network policy, whereas an IDS is an out-of-band device designed as a forensic tool for security analysts.
The differences between IPS and IDS deployment and purpose cause substantial distinctions in prioritizing the requirement, the meaning of the requirement, and implementation options available for meeting the requirement.