The Standard 9x9 Sudoku puzzle has 9 rows, 9 columns and 9 square blocks of size 3x3.
Other sizes of Standard Sudoku are 4x4 (very easy), 16x16 and 25x25 (not so easy).
The Jigsaw variation is the same as Standard Sudoku except that some blocks are not square.
The XSudoku variation is exactly the same as Standard Sudoku with an additional requirement: the two main diagonals must also each contain the symbols 1 to 9 once and once only. KSudoku highlights the diagonals to make this easier to see.
The Roxdoku variations are based on cubes in three dimensions, but are easier than they sound. There are no rows or columns. A 3x3x3 Roxdoku puzzle has 27 small cubes arranged into a larger 3x3x3 cube. This contains nine slices, each containing 3x3 small cubes, and these are the square blocks that must be filled with the numbers 1 to 9. A 4x4x4 Roxdoku has twelve 4x4 slices and a 5x5x5 Roxdoku has fifteen 5x5 slices.
The Samurai Sudoku consists of five Standard Sudoku puzzles of 9x9 squares each, overlapping at the corners by four 3x3 blocks. Each of the five puzzles has 9 rows and 9 columns to solve, but there are only 41 blocks to solve, rather than 45, because of the overlap.
The Tiny Samurai Sudoku consists of five 4x4 puzzles, overlapping at the corners by four squares. Each of the five puzzles has 4 rows, 4 columns and 4 blocks to solve.