What is Resolution - A Graphic Explanation of ppi

From Liz Masoner, former About.com Guide

Photo resolution is one of the most confusing subjects for new digital photographers. What resolution do I need to print a photo? What resolution should web pictures be published? What is resolution anyway?

Resolution refers to ppi, pixels per inch. Sometimes you will hear resolution referred to as dpi, dots per inch. Pixels per inch (ppi) tells you how many pixels are shown per inch on a monitor. 72ppi means there are 72 pixels inside every square inch of display while 300ppi means there are 300 pixels inside every square inch of display. DPI, dots per inch, refers to the same general idea except it is used for printed images instead of projected images. Use ppi on the computer and dpi on paper.

Think of resolution like a batch of marbles. In the photo above the right side of the screen shows the marbles pushed together, there are more marbles per inch of space. The left side of the photo shows the same marbles pushed apart. There are less marbles per inch of space. The marbles did not change size, nor did I remove any marbles. The marbles are simply spread out so that they fill more space because they are further apart.

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