Man in the middle attack is a type of attack in which the attacker intercept into the existing connection and breaks it in two different connections, one between the client and the attacker and the other between the attacker and the server. Now all the data would go through the attacker and he would be able to read, modify and insert data in the communication. Man in the middle attacks are sometimes known as fire brigade attacks.
This attack is very effective on http protocol because data is seny on plain text form. It is possible to capture session cookies by header and also possible to change.
MITM Attack tools
There are several tools to perform a MITM attack.
Cain e Abel