Let's think about network traffic as a cocktail party. Picture Alice and Bob on the love seat chatting it up while Charlie is deep in conversation with Dave at the bar. Meanwhile, Eve is nearby sipping a Hendrix Martini listening in on everyone's conversations.

You see, in order for Alice to send a message to Bob she has to address it to him by his network interfaces MAC address -- or Media Access Control Address. This address is unique every network interface on the planet. Bob's is going to be different from Charlie's, Dave's or anyone else.

On a hub based network, Alice's message is heard by all. But by default when Charlie or Dave hear a message addressed to a mac address other their own, their network interface will drop the frame completely.

This is where promiscuous mode comes into play. If Eve's network interface is in promiscuous mode she doesn't drop frames not addressed to her. This is great for packet sniffing, say if Eve was a network administrator attempting to debug a faulty network. Likewise, if Eve had malicious intent the same applies to eavesdropping.

Now promiscuous mode assumes a hub based network. Switches thwart this by only sending messages to their intended recipients instead of everyone.

Which brings us to Monitor mode. Monitor mode, or RFMON for Radio Frequency Monitor, is one of six modes that wireless network interfaces can assume. Similar to Promiscuous mode, Monitor mode allows the wireless network interface to "sniff packets" not intended for it.

Unline promiscuous mode however, an interface in monitor mode can sniff packets from access points it isn't even associated with. Again this is great for, say, an administrator troubleshooting a network, or on the darker side for malicious purposes such as eavesdropping and cracking encrypted networks.