Taunts

From TF2 Wiki

Each class has its own series of taunts. These can be performed at any time (Default key: G for PC, "X" for Xbox 360 and "Square" for PS3), with each class having a separate taunt animation for each weapon. Each taunt has several lines of dialog that are chosen at semi-random (one taunt is chosen at random and every taunt after it follows a specific list). Taunts are generally used to challenge or insult a player on the enemy team. Exercise caution when taunting. You are vulnerable while performing a taunt as your view will shift to third-person and you will be unable to act until the taunt is concluded. Some weapons do not have taunts, such as the Electro sapper. Only 2 taunts cause damage so far, the Pyro's Hadouken and the Heavy's Showdown, which will cause 450+ damage on a successful hit.

Taunts can also be glitched to produce cool tricks. A popular one is to taunt, then immediately switch to another weapon (usually through the 'use previous weapon' key), so that you taunt with a different weapon.

Taunts also keep you stationary. If you're having trouble with teammates knocking you out of the way, just taunt and you won't be moved.

The Heavy's unlockable Sandvich, when fired also acts as a taunt, moving to third person view and rendering the player immobile. The action is the same when using the taunt key.