Libnids is a library that provides a functionality of one of NIDS (Network Intrusion Detection System) components, namely E-component. It means that libnids code watches all local network traffic, cooks received datagrams a bit, and provides convenient information on them to analyzing modules of NIDS. So, if you intend to develop a custom NIDS, you do not have to build low-level network code. If you decide to use libnids, and you have got E-component ready - you can focus on implementing other parts of NIDS.
