What is sudoku?

Sudoku is a logic number-placement puzzle that was popularized in Japan in 1986 and became internationally known in 2005. The goal is to fill a 9x9 grid of cells, which is divided into 3x3 sub-grids, with the numbers 1 to 9. You begin with some numbers already entered into some of the cells. No digit should be repeated within the same row, column, or sub-grid.