If you design for magazines or books you use digital images. You need images with the right resolution. Digital images are made up of millions of coloured dots and resolution is measured in the number of dots per inch (dpi). When they are printed images must have at least 300 dots for every inch on the page. They can look fuzzy if they are not high enough resolution.
Here is a photograph of the garden of Plas Brondanw near Porthmadog. The first copy is 3,264 pixels wide by 1,840 pixels high. The second copy is 698 by 393 pixels. When you look closely you can see the detail is fuzzy or ‘pixelated’. For example, the railings look strange.
If you wanted to print this picture in a magazine you would need at least 300 dots for every inch of the size in the magazine. 3,264 pixels could be printed up to 3,264 divided by 300 = 10.88 inches wide. 698 pixels could only be printed up to 698 divided by 300 = 2.33 inches wide.
If you had the 698 dpi image but you wanted to crop it and show just the building, what would happen? The cropped picture would be about 300 pixels wide, so it could only be printed 1 inch wide.