nonogram

So here you go: this is my favorite online Nonogram site, here are a bunch more.

It can be used to solve a particular type of puzzle called a nonogram or logigrams.

Edel エデル (Paint by Numbers, Nonogram, Griddler)

This was a reference to the fact that, when the nonogram was complete, the letters inside the picture (a heart, of course) spelled out: ARTTWOTEN.