A “deep packet inspection” (DPI) system is a device that can decode network traffic and look at the contents or payload of that traffic. Deep packet inspection is typically used by intrusion detection systems (IDS), intrusion prevention systems (IPS), advanced firewalls and many other specialized cyber security products to detect signs of attack.
Intrusion Detection Systems can detect and alert, but do not block or reject bad traffic. Intrusion Prevention Systems can block traffic.