See this at: cambridgeincolour.co...| Added on 08/31/06

Since a pixel is just a logical unit of information, it is useless for describing real-world prints-- unless you also specify their size. The terms pixels per inch (PPI) and dots per inch (DPI) were both introduced to relate this theoretical pixel... See more

Since a pixel is just a logical unit of information, it is useless for describing real-world prints-- unless you also specify their size. The terms pixels per inch (PPI) and dots per inch (DPI) were both introduced to relate this theoretical pixel unit to real-world visual resolution. These terms are often inaccurately interchanged (particularly with inkjet printers)-- misleading the user about a device's maximum print resolution. "Pixels per inch" is the more straightforward of the two terms. It describes just that: how many pixels an image contains per inch of distance in the horizontal and vertical directions. "Dots per inch" may seem deceptively simple at first. The complication arises because a device may require multiple dots in order to create a single pixel; therefore a given number of dots per inch does not always lead to the same resolution. Using multiple dots to create each pixel is a process called "dithering." A device with a limited number of ink colors can play a trick... See less