MITM

Man In The Middle. Essentially, an exercise in spoofing two sides of a conversation:

Where a normal connection would go along the lines of

Alice (to Server): I'm Alice, here is my username, password, credit card number, and other data you need 
Server (to Alice): I'm Server, looks good, come in.

A MITM attack looks like this:

Alice (to Bob, pretending to be Server): here is my username, password, credit card number, and other data you need 
Bob (to Server, pretending to be Alice): here is my username, password, credit card number, and other data you need
Server (to Bob, pretending to be Alice): I'm Server, looks good, come in.
Bob (to Alice, pretending to be Server): I'm Server, looks good, come in.

And now Bob knows all the details of Alice's transaction with Server. This is fairly hard to do on a wired LAN, but not impossible. Wirelessly, it's much much much easier, since wireless connections are broadcast everywhere and you can spoof any MAC address you want to. See bsdairtools.