The original Game Boy was released on April 21, 1989 in Japan and in August 1989 in the United States. Based around a Z80 processor, it had a black and green reflective LCD screen, an eight-way directional pad, and two action buttons. It played games from ROM-based media contained in small plastic detachable units called cartridges (sometimes abbreviated as carts).
The game that really pushed it into the upper reaches of success was Tetris. Tetris was widely popular, and on the handheld format could be played anywhere. It came packaged with the Game Boy, and broadened its reach; adults and kids alike were buying Game Boys in order to play Tetris anywhere. It is considered to be a killer game for the Game Boy that made it hugely popular. Releasing Tetris on the Game Boy was selected as #4: Tetris Makes Game Boy a Must-Have on GameSpy's 25 Smartest Moments in Gaming.
The original Game Boy was the first cartridge-based system that supported more than four players at one time (via the link port). In fact, it has been shown that the system could support 16 simultaneous players at once. However, this feature was only supported in Faceball 2000.