I was also confused about the meanings of PPI vs. DPI and so I read a few tutorials on Google (including the top posts on StackExchange network addressing this). I think I know why people are confused even after reading the tutorials:
The term "DPI" is pretty consistently used to refer to printers and printing
However, different tutorials seem to define "PPI" to mean one of three different things:
(a) the meta-information stored in an image file, describing how many pixels should be printed per inch when the image is printed; or
(b) the number of physical LEDs in one inch of distance on the monitor screen -- a physical, unchangeable property of the monitor (or analogously for non-LED screens, whatever other building blocks of light are used for the display); or
(c) the number of image pixels displayed per inch on a display screen; hence, the horizontal screen resolution divided by the width of your screen in inches (which, of course, can be changed)
And this is not casual misuse of the term; this is from people who are writing entire tutorials trying to clarify the correct usage.
(I included examples at the bottom, so you can see the different tutorials really do say different things and I'm not just stupidly misinterpreting them.)
Moreover, when each tutorial gives this definition, they almost never mention the other definition(s) that are in widespread use, which, of course, is probably where the confusion comes from. So doesn't that mean that the tutorials which don't mention the alternative definitions, despite being written with the best of intentions, are part of the problem?
If I wanted to give someone one quick summary that would sort it out once and for all, is the following correct?
Be aware that even among people who are being careful, the term "PPI" is sometimes used to mean any of the 3 things above. (And among people not being careful, all bets are off.)
How an image is displayed on screen by default, is determined by its pixel dimensions -- a 960x540 pixel image will by default take up half the screen in each direction on a 1920x1080 display. It has nothing to do with the image's PPI property and nothing to do with the density of physical LEDs on the screen. In particular, an image with the property of "96 ppi" probably WILL NOT display 96 of its pixels in one inch of the physical screen.
How an image is printed is determined by its pixel dimensions and PPI property, so that an image with the property 96 ppi that is 96x96 pixels will print to exactly one square inch by default.
Now I tried posting the links to the different sources, but StackOverflow said "You need at least 10 reputation to post more than 2 links." So here instead are just the quotes from the different sources about what PPI means (which you can Google if you really want to find the original page for some reason). I'm only posting these to show that the online explanations really do say conflicting things, and it's not just a conspiracy theory I dreamed up:
Votes for "physical, unchangeable property of the screen":
"Pixel stands for “picture element”. It’s the smallest physical element of a digital display device that the eye can discern... Note that pixels are physical things of a fixed size... Hence, the number of pixels per inch (PPI) on your screen is a fixed quantity."
"PPI. Pixels per inch. That’s how many points of light live on an inch of screen."
defining PPI "Same principle: It counts the number of pixels your screen can display per inch." [Note "CAN display", rather than "does display" -- "CAN" display implies it's a physical property of the screen]
[saying that PPI is a property of the monitor] "The images you see on your computer are the result of thousands of tiny colored and illumined squares that physically make up the screen. Those squares are called pixels."
Votes for "image metadata that pertains to printing":
"All that PPI does is affect the print size of the image."
"Pixels Per Inch is a description of the logical number of pixels from your original image that will be used to tell the printer to print one inch on paper."
PPI: "This setting is a single number stored as part of the JPG file, and is used my most programs in determining the scale at which to print the image"
"PPI or "pixels per inch" is the term you will see most often when selecting a resolution for your images in photo editing software."