Picross DS review at Gamepro.

"The basics of Picross are a little hard to explain but I'll give it a shot any way. There is a grid separated into sections; let's say for the sake of demonstration that we have one 15 squares wide by 15 squares high. Each row has a set of numbers attached to it; for our example, let's say one row has 1 2 1. This means that you must fill in one square, then there must be at least one space skipped, then you must fill in two squares, leave at least one blank, then fill in one last square. Which squares you fill in and which squares you leave alone are all dependant on a variety of factors including the surrounding rows and the image that you are trying to draw."

Full Source: Gamepro