Butterfly Sudoku consists of four overlapping 9x9 Sudoku grids, arranged in a pattern that resembles a butterfly.
Because the grids overlap, some cells are shared between multiple grids. This means that the digits placed in these shared cells must satisfy the rules of all the grids they belong to.
Essentially you are solving 4 sudokus at the same time, with overlapping sections.
Butterfly Sudoku overlapping nature of the grids is the defining characteristic. When solving, you must pay close attention to the shared cells, as they provide crucial clues for solving the entire puzzle.