When and Why to Go All-In

When and why to go all-in?
While the reason to go all-in seems to be quite clear, when to do it depends in most cases on the player's style, his knowledge of the game and the goal he has set himself (e.g. to finish in the money or bet on first place).

While in the cash-game the dynamics are different as the deepness of the stack is always around 100 big blinds or more, in tournaments the situation changes because the level of the blinds progressively rises, leaving more or less space for the postflop.