How To Play Ken-Ken
The only numbers you may write are 1, 2, 3, or 4. (A 6×6 puzzle requires 1 through 6.)
No numbers may appear more than once in any row or column. (That is, all required numbers must appear in every row and column.)
Each “cage” (region bounded by a heavy border) contains a “target number.” If there’s more than one cell in the cage, the target is also accompanied by an arithmetic operation. You must fill that cage with numbers that produce the target number, using only the specified arithmetic operation. Numbers may be repeated within a cage, if necessary, as long as they do not repeat within a single row or column.
In a one-cell cage, just write the target number in that cell.
(From http://thinkmath.edc.org/resource/introducing-kenken-puzzles )