Man-In-The-Middle: An attack in which communications between two hosts are routed through the attacker's host. The attacker can observe, modify, and/or block selected traffic before relaying to the intended host. Communications between the target hosts appear normal.

Replay: An attack in which a copy of communications between two hosts is obtained by the attacker. The attacker retransmits selected portions of the copied communications at a later time for nefarious purposes such as creating duplicate transactions, circumventing authentication, etc.

diffrent: a replay attack is a more specific type of man-in-the-middle attack. I view the biggest difference in the intent. In the man-in-the-middle attack the intent is simply to capture the data, but in a replay attack the intent is to reuse the data in an an attack.