The Grid
A standard Sudoku is composed of a 9x9 grid (81 cells). Each cell can be either empty, or filled with a digit between 1 and 9.
This grid is further divided into sections (also known as boxes) of 3x3 cells as shown in this image. For a Sudoku to be correct only one instance of each digit can be present in each row, column, and section.
Even though a standard Sudoku use digits, there is no math involved, and those digits can be replaced with other symbols, such as characters or abstract symbols. The only requirement is that there are 9 unique symbols.
The Givens
For a Sudoku to be solvable only by logical reasoning with no guessing, a number of digits need to already have been placed. These digits are called Givens, Clues, or Fixed and can not be changed.
The Pencilmarks
To aid in solving a Sudoku, one of the most important techniques is marking the cells, which is called pencilmarks, or notes. It consists of marking each cell with small digits or dots in specific positions to aid which digits can be placed in each cell. This makes it easier to use further techniques to eliminate candidates and find the correct solution.
