Sudoku is a logic-based, combinatorial, number-placement puzzle. The objective is to fill a 9×9 grid with digits so that each column, each row, and each of the nine 3×3 sub-grids that compose the grid (also called "boxes", "blocks", "regions", or "sub-squares") contains all of the digits from 1 to 9. The puzzle setter provides a partially completed grid, which for a well-posed puzzle has a unique solution.
Completed puzzles are always a type of Latin square (square filled with n different symbols, each occurring exactly once in each row and exactly once in each column) with an additional constraint on the contents of individual regions. For example, the same single integer may not appear twice in the same row, column or in any of the nine 3×3 subregions of the 9x9 playing board.
source & more information: https://en.wikipedia.org/wiki/Sudoku