1. Document Size
1.1. Document Size goes hand in hand with Pixel Dimensions, yet is also completely separate from it.
1.1.1. Notice at the bottom of the Document Size section, it says "Resolution", and in the Resolution box, it says a number such as "72". Notice also to the right of that is another box, this one saying "pixels/inch".
1.1.1.1. What this is telling us is that when we go to print the photo, 72 pixels out of our 3456 pixels from left to right in our photo (the width), and 72 pixels out of our 2304 pixels from top to bottom in our photo (the height), will be printed for every one inch of paper.
1.2. That’s what "image resolution" means – how many of your image’s pixels left to right and how many of the pixels top to bottom will print in every inch of paper.
1.2.1. All "image resolution" means is how many of your image’s pixels will print inside every inch of paper. It has no effect at all on how your image appears on your screen, since your monitor has nothing to do with your printer.
2. Pixel Size
2.1. An inch is an inch is an inch
2.2. There’s one other aspect that image resolution has to do with, and that’s the size of the pixels when you go to print the image
3. What is it?
3.1. It is the width and height of our image in pixels. It is the amount of pixels are in the image from left to right, and from top to bottom.
3.2. It’s also telling us one other important piece of information which is the file size of our image.
3.3. MP
3.3.1. The "MP" stands for "mega pixel", and "mega" means "million", so "8MP" means 8 million pixels.