Dots Per Inch is a method of measuring a picture. A picture's width and height in inches is determined by dividing the dimensions in pixels by the DPI of the image. For instance, a 3,500 by 3,530 pixels image at 72 DPI is about 48.6 by 49 inches.
Changing the DPI of an image without resampling the image results in a different physical output size but retains the quality of the image. For instance, changing a 3,500 by 3,530 pixels image from 72 DPI to 300 DPI shrinks the physical output size of the image from about 48.6 by 49 inches to about 11.7 by 11.8 inches. Color newspaper advertisements typically require between 150 to 200 DPI, glossy magazines typically require 266 to 300 DPI, high-end books can require as much as 350 DPI and consumer inkjet printers usually require at least 150 DPI.