Arp cache poisoning puts the attacker in position to intercept communications between the two computers. Computer A believes it is communicating with Computer B, but because of the poisoned arp table, the communication actually goes to the attacker’s computer. The attacker can then either respond to Computer A (pretending to be Computer B), or simply forward the packets to its intended destination, but only after the packet information is captured and logged for later use by the attacker. Likewise, the response from Computer B can be captured and logged by the attacker, who has also used Arp poisoning to make Computer B think the attacker’s computer is Computer A. This type of attack is known as Man in the Middle attack.

via Arp Cache Poisoning and Packet Sniffing – ADMIN | The resource for all system administrators.