A Nonogram is a grid with numbers along the top and the left side. Shade the right squares and a picture appears. It's pure logic: no maths, no guessing.
Each number is a run: that many filled squares in a row, with no gaps. If a line has more than one number, the runs come in that order, with at least one empty square between them.
If the runs plus the single gaps between them add up to the full width, there's only one way to place them.
Solve a row as far as you can, then look at the columns those squares belong to. Each filled square or cross is new information for the column. Go back and forth until the picture appears.
Yes. Nonograms are also sold as Picross, Griddlers, Hanjie, Paint by Numbers and Pic-a-Pix. The rules don't change.
That row or column has no filled squares at all.
No, they're optional notes. But marking squares you know are empty makes the next step much easier to see.