I've heard three terms for three types of Medic.
The first one is 'Passive Medic;' A Medic who just runs around and heals people. Gets plenty of assists, but isn't good at saving himself when in danger.
The next two I've seen interchanged, but I've mostly heard them used like so:
Combat Medic: A Medic who heals his team as much as possible, but isn't afriad to take out his gun against a vunurable sentry in Doublecross or someplace. (We've all taken out a lvl.3 sentry as Medic. It's pathetically easy.) He'll use his needles when they are more use to him than a team mate. A really good combat Medic will use the crossbow and heal/kill from across the map.
Battle Medic: This is what everyone despises. He's the Medic that will charge out with the Blutsauger with the thought in his head that it's as powerful as a scattergun. He'll seldom get kills and rarely heal his team.
There are good battle Medics though. I've been a good battle Medic before, and there is nothing more rewarding than seeing; 'RED Medic is a fucking monster' in the chat. The key to being a good battle Medic is to equip the head and Blutsauger, then play in the most seldom traveled parts of the map. Lead your targets, and run backwards. They'll fall to your aids-filled needles in a heartbeat.
The last and best type of Medic, which is more of a branch off the battle Medic type, is the chain ubering Medic. These two will fuck shit up, even for a team consisting mostly of Pyros. A chain uber is really hard to pull off, and even the best Medics will find it hard. But it reaps rewards and dominations, it is worth the effort.