Sudoku is a number game in which missing numbers are to be filled into a 9 by 9 grid of squares which are subdivided into 3 by 3 boxes so that every row, every column, and every box contains the numbers 1 through 9.
In other words, given a nine by nine grid with some squares are pre-filled with numbers, you need to fill in numbers to all empty squares so that every number 1 through 9 appears once and only once in each row/column/box.
A classic Sudoku has one and ONLY one solution, and two squares that are symmetrical around the center square are either both filled with numbers or both empty.
Sudoku is American invented. Howard Garns created it as Number Place in 1979 but died in 1989 before Japanese publisher Nikoli got a hold of it. Sudoku became mainstream in 1986 by the Japanese puzzle company Nikoli, under the name Sudoku, meaning “single number”. The game didn’t really take-off until 2004 though when Wayne Gould convinced The Times in London to publish it.
There are many online Sudoku websites offering free Sudoku puzzles, such as websudoku.com or New York Times Daily Sudoku. There is a special online Sudoku website which, besides of billions of Sudoku puzzles, shows you how each of their Sudoku was made, and how each of Sudoku puzzle could be solved.