F5

F5 aka Third Person Mode or Toggle Perspective is a method which a player can switch themselves into the third person perspective, by pressing the F5 button on their keyboard. Some players who use this function will keybind the button to another more convenient to press, though the community calls it by it's default key when referring to it.

Functionality
F5 is used to change the perspective of the player to third person, which can be for a number of reasons, including, but not restricted to: F5 toggles between three perspectives, first person (default view of minecraft), third person view from behind their character's head, and third person view from in front of their character's face.
 * Taking screenshots
 * Better view of their character animation or skin
 * To see around corners and over walls

Controversy
Because of the last reason F5 is used, that coupled with the fact that this is a Ranged-based combat game, people began to realize that F5 would give people an unfair advantage (Similarly to F3+B). Over time players began to dislike the play style, and today players who use this mode are generally shunned, as well as F5ing is banned from tournaments (notably CvC Open and CvC International) as most players want CvC to be an FPS like Counterstrike (the original source for the game).