Nurikabe is a puzzle where you fill in black squares to create separate groups of white squares. The black squares must all be connected in a single black shape (diagonally doesn't count). Groups of white squares can be any shape. White squares that only touch diagonally are not in the same group. A group of white squares has to have exactly one number in it, and the number of white squares in the group has to equal the number in it.
Go to Nikoli.com to learn more about how to play Nurikabe.