Many things can be sayed about pong. It is certainly a game which changed the video game industry.
Contrary to what most people think, Pong is not the first videogame nor it is the first arcade video game (Computer Space, 1971). It is however the first successful arcade video game which really started the videogame industry.
Instructions were simple: "Avoid missing ball for high score" and that was one of the main reason for its success.
However, PONG was not really designed by Atari. As a matter of fact, Magnavox was putting their new Odyssey home video game console in demonstration. On May 24, Nolan Bushnell (Atari founder) went to one of these demos at Burlingame, California where he signed the guest book and played the Tennis game, a simpler version of Pong.