Tetris battle is the predecessor of Tetris. It is the same style as tetris, but it is made for the purpose of playing tetris against your friends, instead of alone.
It is as 1 vs 1 style of game, that takes tetris into a competetive game. The same rules of Tetris applies, but instead of just getting a higher score, you send the lines that you clear to your opponent. If you clear the lines, you will claim them as their own, and add those lines to your own total amount of cleared lines. If you fail or your opponent doesn't clear their lines, when it reaches the top of your screen, then you gain a "ko". Also called "knock-out". In regards of the total amount of cleared lines, KO's are more valuable.