To produce shades of black, anything that has to be printed must first be broken up into a raster. The size of the black dot and the white paper around it create the impression of greys.
Light areas have small dots and darker areas or shadows have larger dots.
The image broken up by the raster is called a halftone, the raster is a halftone grid, divided into halftone cells. The halftone grid in the illustration above is dependent on the fineness of dots the output device can produce. In other words, if you have a 300 dpi laser printer, your grid can only be produced using 300 dots per inch, which means that you are limited in the variance of your halftone dot, much more than with a 3600 dpi imagesetter. This limitation means that the distribution of your halftone cells depends on your grid, if your cells are too close together, the grid will not allow the printer to vary the size of your halftone dot and you cannot produce enough shades of grey, the image becomes blotchy. The closeness of the rows of halftone cells is measured in lines per inch or the line screen.