What are the kinds of sudoku?

SUDOKU or Single Number, is a Japanese number puzzle that has swept through the world with the same gusto as the Rubiks Cube. Newspapers carry it, software houses, toyshops and bookshops sell a great deal of it. There is even a world championships. In the basic form it is a nine-by-nine grid divided into nine (sub-set) squares of nine squares. Numbers between one and nine are strategically placed by the compiler and the player is invited to fill in the rest. The rules are very simple, the game is incredibly addictive. Fill in all the squares with one number each of one to nine. There are to be no repeated numbers in any vertical or horizontal line nor a repeated number in any sub set (square) of nine squares. How hard can it be? Common variations of Sudoku are 8x8, 12x12 and 16x16 squares. Letters or shapes can be substituted for numbers. Other Japanese number puzzles include Kakuro, Kokonotsu, Kikagaku, Samu.