What is PPI : Pixels Per Inch

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Pixels Per Inch is a metric used to describe the pixel density (sharpness) for all sorts of displays, including cameras, computers, mobile devices, etc…
A Smartphone display has thousands of individual pixels. Pixels stand for picture elements – they’re the basic building blocks on a smartphone display. Images are formed by directing the millions of pixels to change colour in unanimity. By changing each pixel to give the desired colour, smartphones can produce almost any image.
It is important to understand what it really means in a world where visual computing and visual quality has increased exponentially over the past decade, and where PPI has become a prime marketing tool.
PPI is a pixel metric, but it can't be used by itself as a sharpness benchmark because the distance between our eyes and the display is as important as the pixel density itself. If you bring your screen closer to your eyes, you will see the pixels.
If you move the device further away, the additional pixel density may not be useful at all, because it won’t be detectable. 
Smartphones are used much closer to your eyes than tablets. Computer monitors are a little further away and TV, cinema screen etc are even farther. Because of that, they require different PPIs to achieve the same perceived sharpness from your point of view.

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