What is a pixel, what does DPI mean? There’s a lot of confusion in this area, to this day, among traditional print designers when they’re working with the web.
Let’s start simply, with inches. One inch is approximately 2.54 centimetres! When computers look at images, they don’t know anything about inches, all they know about is dots. Those are called pixels. On the internet, the concept of inches is completely irrelevant because, to a computer, your screen is measured in pixels. Things can get a bit confusing here because screen pixel sizes are often called resolutions, which is not the same as the DPI sense of resolution.
So, let’s recap…
Inches - Used in the real world, irrelevant to computers and the web
Pixels - Dots, the way computers see images (lots of dots)
Now we introduce the complex part, DPI which literally means Dots Per Inch. It’s actually that simple. If you have a computer image which is 300 pixels by 300 pixels and it is then printed onto a physical piece of paper at 1 inch square, your Dots Per Inch is a simple calculation of 300/1 which equals 300dpi. The higher the DPI, the more computer dots per phsyical inch, and in most cases, the higher the quality of the printed output.
But if you take the physical printing (or scanning) out of the equation, inches no longer apply, and thus DPI no longer applies.
See, it’s simple when you know how… 
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