An OSB file is a storyboard file created by the rhythm game Osu! that lets players tap visual elements on screen with the beats of music. The storyboard inside the OSB file contains a customized animated background that is used throughout behind the game while playing. The background of storyboard is typically a JPG or PNG file that is shown with the OSZ beatmap file during the game.
OSB File Format
OSB files are stored as plain text files to the disc in beatmap folders. This scheme allows to create different storyboards having different difficulties within a beatmap.