The classic puzzle game by Alexei Pazhitnov, considered to be the defining game on the Game Boy!
Tetris gives you a screen which slowly fills up with "Tetrads" made up of four blocks in various shapes. You have to move and rotate the blocks so they form solid rows across the screen, whereupon the row will vanish and you will score points. Clearing more than one row at a time will net you a bonus multiplier. If the screen fills up with blocks then the game ends.
In Game A you play endlessly for high scores. Acquiring a large high score may net you one of several rocket animations available. The better your score, the bigger the rocket that takes off.
Game B requires you to clear a certain number of lines in order to win the game. You can increase the number of lines you need to clear, making the game longer.
For both games you can increase the number of blocks already on the screen when you start, as well as the speed. There are three kinds of music to choose from as well. If you have a Game Boy Link Cable you can play a multiplayer game, where indestructable rows of blocks rise from the bottom of the screen when your opponent clears lines.