Intrusion Detection System (IDS) monitors policy violations or malicious activities in a system or network. It could be a device or a software application.
IDS differs according to the size of the network. There are two main classifications: Host-based Intrusion Detection System (HIDS) and Network Intrusion Detection System (NIDS). HIDS observes important Operating System (OS) files. While the NIDS analyses network traffic, they are placed at a unique point or several unique points to monitor outgoing and incoming traffic within the network.
Hence it tries to identify unique sequences appearing across several packets in a traffic stream than in individual packets.
