Sudoku is a simple game of logic, whose name originated in Japan. The object is to complete the grid such that every row, every column, and every 3x3 block contains the digits from 1 to 9. There is no mathematics required to solve the puzzles, just pure logic and reasoning.
The rows and columns are labelled 1 to 9, as are the 3x3 blocks. Each cell is uniquely referenced, e.g. R2C5, for example, means the single cell in Row 2 and Column 5. Also, e.g. Block7 is the block numbered 7.
A completed Sudoku will look something similar to the following:
As you can see, each row, column and 3x3 block contains the numbers 1 to 9. The 'buddies' of a square are the squares that share a row, column or 3x3 block. e.g. the buddies of R3C7 are shown below. Every square has 20 buddies.