18 Puzzle is a variation of classic 15 Puzzle game. Not only does 18 Puzzle has 18 tiles to move (vs. 15 in classic Fifteen puzzle) but also 18 Puzzle uses hexagonal board making the game more challenging. The object of the puzzle is to place the tiles in order starting from 1 in the top left corner to 18 in the bottom right by making sliding tiles into empty space.
If 18 tiles is not challenging enough for you, you could always increase the size of the board and number of tiles to move, or the other way around if 18 tiles is too much for you - make the board smaller. All in all the game is not just one but three puzzles (and even 4 on the iPad): "6 Puzzle", "18 Puzzle" and "36 Puzzle".
Finally, 18 Puzzle is integrated with Facebook. Once Facebook integration is activated the time it took you to solve the puzzle is posted to your Facebook wall so you could brag how fast your brain is and challenge your Facebook friends.
Enjoy