WHat does that 'pixels per inch' mean exactly?

I'm using PhotoShop CS to edit some pictures in order to create wallpapers at 1024x768 pixels.
I used to have the notion that more pixels per inch mean a better resolution of a particular image. However I observe there are bmp and jpg files that have different pixels per inch, some have 72, other have 200 or 300. By using PhotoShop and loading and saving the same files while increasing the number of pixels per inch, I don't see any difference in picture quality.
I even saw pictures at the same size (1024x768) at 72 dots per inch having better quality than some others that have 300 dots per inch.
Please any graphics expert explain this 'pixels per inch' definition before I'm getting lost?
VolgaAsked:
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Lee W, MVPTechnology and Business Process AdvisorCommented:
The quality of the image won't change - the SIZE of the image will.
http://en.wikipedia.org/wiki/Pixels_per_inch
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VolgaAuthor Commented:
Thx for the link. However it still doesnt' explain that I have 2 identical images of the same size (1024 pixels x 768 pixels), one with 72 pixels/inch, the other with 300 pixels/inch. In terms of quality they are exactly the same on the screen.
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Lee W, MVPTechnology and Business Process AdvisorCommented:
Yes, they are the same - as I said - the size of the image will differ.  Sorry if I wasn't clear - the SCREEN size will be the same (the screen has a set number of pixels and doesn't care what the "per inch" rate is - but try to print them.
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