Intrusion detection systems (IDSs) are available in different types; the two main types are the host-based intrusion system (HBIS) and network-based intrusion system (NBIS). Additionally, there are IDSs that also detect movements by searching for particular signatures of well-known threats.

An IDS compliments, or is part of, a larger security system that also contains firewalls, anti-virus software, etc. A NIDS tries to detect malicious activity such as denial-of-service attacks, port scans and attacks by monitoring the network traffic.