Also known as Nonograms, this is the pixel art among the puzzles.
The numbers in the puzzle tell how many adjacent squares should be coloured. By colouring that what you know, and crossing out the spots where you are sure it can't be coloured, you gradually solve the puzzle: a pixelated piece of art.
I love these puzzles and created an program to create and play some puzzles myself, both of which you can find here. If you need some hints on how to solve picross puzzles, take a look at my tutorial.