It's a logic puzzle using numbers to create patterns where every line and numbers one through five in it with no repeats and all nine boxes have no repeats. You only get a certain amount of numbers put in place for you and then you have to fill in the rest yourself. Usually the harder the it, the less numbers you get. Some games are timed, too.
[It is sad you don't know what Sudoku is, Kitty.] But it was supposed to be easier.