As you wrote it, the answer is that there is no difference (until you print it or look at it in a document that will be printed).
First a clarification: PPI is pixels per inch, a description of the resolution of the image.DPI is dots per inch, a description of the physical ability of the printer/scanner being used.
pixels (on a side) = ppi x inches.multiply the two sides to get the total size of the picture, usually measured in MP (megapixels).
My local paper runs a section where people can submit their photos, and the instructions are that the pictures "must be at least 300 PPI". I've always been tempted to turn in a 300x300 pixel image at 300 PPI. This of course is low-res (.09 MP) and only prints at 1"x1", but would meet their posted requirements.
The assumption behind statements like this is that the image is a "reasonable size" when printed (like 4"x6" for example). In that case, 300 PPI means it would also look sharp (contain a lot of pixels, in this case 1200x1800=2.2MP), compared to 72 PPI which would look, well, pixelated at .1MP.
In your example they are measuring by pixels ("1200 pixels on the long edge"), not in inches, so then the PPI value is immaterial.
