At a functional level, an NMIC may differ from a NIC, in that the NMIC may not have a MAC address, may lack the ability to transmit and may not announce its presence on a network.
Advanced NMICs have features that include an ability to offload CPU intensive processing from a system's CPU, accurate time measurement, traffic filtering, and an ability to perform other application specific processing.
This is a separate function from NMICs that are used for data collection and processing.
NMICs are typically used in intrusion detection and prevention (IDS/IPS), lawful interception, flow analysis, network monitoring, and protocol analyzer systems.
Notable manufacturers include Endace, SolarFLare and Intel.