Explanation: An intrusion detection system (IDS) is implemented to watch network traffic and operations, using predefined criteria or signatures, and alert administrators if anything suspect is found. An intrusion prevention system (IPS) is similar to an IDS but actually takes action against suspect traffic, whereas an IDS just alerts when it finds anything suspect. A firewall works at the network level and only takes into account IP addresses, ports, and protocols; it does not inspect the traffic for patterns or content. A web application firewall (WAF) works at the application layer and provides additional security via proxying, filtering service requests, or blocking based on additional factors such as the client and requests.