A man-in-the-middle (MITM) attack occurs when an attacker inserts himself between two devices and is able to read, insert, modify messages between the two devices.
The most difficult part of a MITM attack in wired networks is getting in the middle without being detected. Usually this requires physical access to the network increasing the chances of being discovered.
In wireless network an attacker can insert his device in the the path of communication remotely and never expose himself making this one of the most dangerous types of wireless attacks. A MITM attack can be used to break connections such as SSL, SSH and VPN.
A wireless MITM utilizes a rogue access point, rogue station and phishing to exploit a user connected to the wireless network. Usually the rogue access point is implemented as a software based AP using a PC with dual wireless network interfaces.