The Ubercharge is a special ability from the Medic that can be recognized by it's reddish or blueish projectile beam. It's special ability by default is to grant the subject that is Ubercharged to be invincible until the Ubercharge runs out after the 8 seconds with the default Medigun. Ubercharged players can be recognized by their blueish/reddish aura (depends on the team color). There are different ubercharges for each different medigun.
Heavy and Medic with Ubercharge.
The medigun's ubercharge is the default, being a default/stock weapon giving the user and the medic 100% invulnerability to all player attacks, but not to environmental attacks such as the sawblades on Sawmill, the train on Well, and pitfalls. This does not grant invulnerability to Telefraging, and still kills the medic or the patient instantly. This is best used to take out a sentry nest when no good spies are available, a large group of enemies, a push, or to stop a enemy push.
The kritzkrieg's ubercharge gives the target and the medic 100% critical hit chance on all weapons that have a critical attack. It is best used to ambush a large group of enemies or to combat a enemy being healed with another kritzkrieg ubercharge or a quick fix ubercharge. Remember that using this on buildings is not a good idea as all buildings don't take extra damage from critical hits or mini-crits with no exceptions. Do not use this ubercharge on a soldier that is wielding the Cow Mangler 5000, as it does not get critical hits under all circumstances. This weapon also has a 25% faster ubercharge rate. The downside is that players are still as vulnerable to damage as they would be before ubercharge, and can lost quickly to a spy if they are not careful. The 25% faster ubercharge rate lets a medic deploy this more often, as a crit-boost is less likely to save your life than a guarenteed invulnerability from the default medigun.
The quick-fix's ubercharge is healing the medic and the patient being healed as if a medic were healing another medic with the quick-fix 300% faster than a usual quick-fix would heal a person, which is 40% faster than a regular medigun would heal. This ubercharge can stop enemy pushes, and destroy a medium-sized sentry nest, as the healing does not make the medic or the target invincible. Due to the slightly faster ubercharge rate (only 10% faster), the ubercharge of this weapon can be used frequently than a regular one, as the effects are more for general effects, such as a almost dead patient fighting a few enemies that would not be a problem at full health but are at low health. The downside is that concentrated fire can easily take out a person on this ubercharge, or even a lone heavy using any Minigun, even the Tomislav at point blank range. Even spies can quickly eliminate quick-fix players. However, due to the faster charge, dying with full ubercharge is not a big concern with the faster ubercharge rate.
The vaccinator's ubercharge defends the medic and his patient from 75% damage of the selected damage resistance, which can be fire, bullet, or explosive. Melee damage is counted as bullet damage, and drowning is not affected. This ubercharge builds 50% faster, letting most players use it without worry of a waste as it builds much faster than usual. The selected damage resistance cannot be changed during ubercharge, forcing a competent player to be careful over which damage resistance chose before ubercharge. This ubercharge can be activated at 25, 50, 75, and 100% ubercharge rate, unlike any other current medigun. The charge will last 2, 4, 6, and 8 seconds long. The downside is that the targets can still be killed with ease from other sources of damage if attacked by multiple classes, and Backstabs are still a major threat. The medic is healed 25% of the damage you take, an example is if a person under a fire resistance ubercharged is hit directly with a grenade that does 100 damage, the medic is healed 25. This will not give Overheal to the medic. Also, all critical damage is forced to do regular damage for the selected damage resistance type. Another downside is that the ubercharge will lose 3% of it while active to all sources of damage, with no exceptions making bullet resistance againts a Heavy at medium range almost pointless as the charge will run out very fast, even at 100% charge. Dying is almost not a concern, as 50% faster ubercharge rate means that dying with full uber really only wasted at most 30 seconds, and you still healed people anyway.