KenKen is the hottest puzzle craze since Sudoku. It combines the logic of a Sudoku puzzle with arithmetic. You have to add, subtract, multiply and divide to arrive at the numbers given in KenKen puzzle.
In KenKen, like Sudoku, there is a grid of 4x4, 6x6, 8x8, or 9x9 cells. That means the grid has 16, 36, 64 or 81 cells. The puzzle can actually be even larger, but those are for really advanced solvers. In both kinds of puzzles, you must have all the digits in one row. In other words, if a row has 4 cells, like in a 16-grid puzzle, you must use the numbers 1-4 in each row, and in each column. No digit may be left out of row or column, nor may there be duplicates in a row or a column.
So far, that's totally easy to understand. If you look at a puzzle, you can see what I mean in a second.
The diabolical cleverness of KenKen, which sets it apart from Sudoku, is that: