The President of Earth is the ruler of the entire planet Earth, supposedly during a period occurring after all countries of the world have agreed to merge their governments.
In many cases, the President of Earth is portrayed as occupying an office very similar in style to that of the President of the United States, perhaps alluding to an eventual American takeover of the world, or simply a pro-US bias of the author.
Presidents of Earth have been characters in:
In Futurama, Richard Nixon's preserved head becomes the world's president.