Nonogram
A nonogram is a grid puzzle in which numbers along the edge of each row and column give the lengths of the consecutive runs of filled squares in that line, and shading the correct squares reveals a picture.
A row labelled 4 2 contains a run of four filled squares, then at least one empty square, then a run of two, in that order — but the numbers say nothing about where along the row those runs begin. Every square you can prove must be filled, and every square you can prove must be empty, constrains the crossing line, and the two directions feed each other until the image resolves.
The standard first move is the overlap. In a line of ten squares, a single run of eight can start at square one or square three at the extremes, and the middle six squares are filled under either reading, so they can be shaded immediately. Marking proven empties with a dot or a cross matters as much as shading, because an empty square is what separates two runs.
The form was devised independently by Non Ishida and Tetsuya Nishio in Japan in 1987. It also travels under the names Picross, Griddlers, Hanjie and Paint by Numbers. A properly made nonogram has exactly one solution and can be reached by deduction alone, with no guessing at any point.