A Man-In-The-Middle (MITM) attack is achieved when an attacker poisons the ARP cache of two devices with the (48-bit) MAC address of their Ethernet NIC (Network Interface Card). Once the ARP cache has been successfully poisoned, each of the victim devices send all their packets to the attacker when communicating to the other device. This puts the attacker in the middle of the communications path between the two victim devices; hence the name Man-In-The-Middle (MITM) attack. It allows an attacker to easily monitor all communication between victim devices.

The objective of this MITM attack is to take over a session. The intent is to intercept and view the information being passed between the two victim devices.

Reference: http://www.cisco.com/c/en/us/products/collateral/switches/catalyst-6500-series-switches/whitepaperc11_603839.html