There is a nice article I found explaining dots, pixels, ppi, dpi etc. Here it is. Very good overall, but I got confused at one point where it says:
For example, if you are printing a 150ppi image at 600dpi, each “pixel” will consist of 16 dots (600 dots/150 “pixels” = 4 rows of 4 dots per “pixel”).
Is this true? From what I gather it should have been 4 dots / pixel. Why "4 rows of 4 dots per pixel". Where did those rows came from? Is there something I'm missing? At any rate, I believe that it should be 4 dots / pixel. What is right here?