DPI (dots per inch) vs PPI (pixels per inch)
DPI and PPI are the same general idea. When you compare DPI vs PPI, you learn they are both a measurement of density that reflect the quality at which an image is being displayed. Both DPI and PPI are based on the concept that you can take a whole bunch of small pieces to render a much more complicated image. PPI is typically referring to digital media using pixels. Each pixel in digital can be one of 65+ thousand colors, allowing for a lot of flexibility. When you are talking about printers or DPI, it’s a slightly different story.
Working with printers you typically only have 3 or 4 colors. Those colors have to combine to fake more than 65 thousand colors that digital displays can display. How do they do it?
It isn’t as pretty, but if you used a magnifying glass to look at your photograph from an ink jet printer you’d see it was using the same methods as this halftone picture of a light bulb. Up close you see something similar to the golf ball picture using pixels, only each dot is made up of yellow, cyan and magenta and black. Just as you see all the dots in this half tone photo you would see all the dots from an inkjet with a magnifying glass. As you back away from each you no longer see the dots but instead you see the photograph as a smooth composition. Take a few steps away from your computer to see what I mean.
The photograph of the golf ball has been enlarged to show the individual pixels. When pixel density is around 72-150 pixels per inch as they probably are on your screen you can’t even notice these little squares. Same story when you print a photo with 150-300 pixels per inch. However, the quality or “smoothness” of the image increases as you get a higher density. If you ever look at a retina display you’ll notice that the quality is much higher than an older Apple display. That’s because there are 300+ pixels per inch on a retina display.