DPI what PPI do these mean

Nowadays mobiles, tablets, television, etc all have high resolution displays. We can know the number of pixels on display by its resolution. When people watch specs on various devices they often get confused with DPI and PPI of the display

Lets first understand the pixel
A pixel is the smallest unit from which an image is made on a display. An pixel is made up of 3 sub-pixels red, green, blue.

What are dots then
These are the smallest unit by which a printer prints image. An printed image is made up of tiny dots

PPI (pixels per inch) : defines number of individual pixels present per square inch (2.34cm) of a display

DPI (dots per inch) : it is similar to PPI in this case individual dots and it refers to printers

Lets take nexus 5 for understanding:-

Nexus 5 has a 5.2 inch display with a resolution of 1080×1805. Which means horizontally the phone has 1080 pixels and vertically 1805 which gives nexus 5 a PPI of 445

But sometimes DPI is used to describe the quality of displays, which is misleading. DPI is the measure of how much dots there are in a printed picture and cannot be applied to display as they are made up of pixels.

For eg: if you want to print a picture of a resolution 1920×1080
If you print using 150 DPI you will get 150 individual dots in an inch which will make up the image

But if you print the same image using 300 DPI you'll get 300 dots to make up the image. So the image with 300 DPI will look sharp as the dots are printed close to each other which makes them hard means distinguish.