Medic
The medic is a maniacal German scientist who is capable of healing his teammates and making them invulnerable for short periods of time. In 6v6, the medic makes up a key component of the combo, along with the pocket soldier. In highlander, the combo usually consists of a medic, heavy, and demoman. As the medic does not need to focus on aim, medics generally act as callers for their teams. Medic duties generally include keeping the team fully buffed, timing and leading pushes, and tracking enemy ubers.
6v6
Medic is most commonly thought of as the most important class in all of TF2. He leads the pushes in order to cap more points, and in order to destroy the enemy team. His job is to give buffs to every class. Buffs are a health boost that all classes can receive. Soldier can be buffed from 200 to 300, Scout from 125 to 185, etc. etc. In 6's the medic usually uses two different types of medi-guns. The original medi gun, which makes the player that recieves the uber invincible, and the kritzkrieg, which gives 100% kritz to the player receiving the uber. The kritzkrieg is usually given to the demoman in order to drop the other teams medic and t gain a large advantage. The demoman receives the kritz because he has the largest damage output and has weapons that best suit the kritzkrieg. The medic has a pocket soldier who's main objective is to keep the medic alive. He receives most of the ubercharges from the medic. Keeping the medic alive is very iomportant and the team with the better medic usually has an advantage.
Highlander
Main Article: Medic (Highlander)