In networking terms, a computer having its network interface card set to “promiscuous mode” receives all packets on the same network segment. In “normal mode,” a network card accepts only packets addressed to its MAC Address.
When the network card is in “promiscuous mode,” it not only accepts all of the packets on the same network segment but also passes them to the OS. This process is helpful for capturing passwords, monitoring networks, and finding malicious packets. Using sniffers, system administrators routinely check whether any network interfaces are set to “promiscuous mode” to discover possible intrusions.
Eyeonsecurity. About Sniffers—Their (ab)use in Networks. [Online, 2004.] Eyeonsecurity Website. http://eyeonsecurity.org/articles/sniffers.html.