Samurai Sudoku ("Gattai-5") is one of the sudoku variation with overlapping sudoku grids. The puzzle consists of five grids, one in the center and the other 4 overlapping each corner grid of the central one. This means that the numbers must be placed correctly for all the five puzzle squares.
You're solving five Sudoku puzzles at once, but they are interconnected, making the puzzle much more challenging and interesting.