Taiko is a special game mode in osu! that borrows almost all of it's elements from the Japanese rhythm game, Taiko no Tatsujin (Released as Taiko: Drum Master in North America).
In this game mode, players hit the drum (or keyboard) in time with blue or red notes on the screen. Red notes indicate that you must hit the middle of the drum (x or c on keyboard controls) and blue notes indicate that you must hit the rims of the drum (z or v on keyboard controls). Large red or blue notes indicate that you have to hit both sides of the drum, middle or rim according to the color of the notes. Long yellow notes indicate a roll of either red or blue notes. These do not add to your combo or cause a combo break if you choose not to roll throughout the entire thing.
In the osu! variant of Taiko, red balloons and bells found in some versions of the console game have been removed and replaced with a shaker that mimic's the osu! spinner. This shaker requires the player to alternate between rim and middle to whittle down the number on the screen before the next set of notes appears. Unlike some other rhythm games, players can play random notes to the beat without fearing a point deduction, as long as other notes are not on the track.