Pixels Per Inch is PPI, this is the density of images in digital medium, or in your computer.
DPI is Dots Per Inch, and this is a printing term. So, for instance if you had a really huge image with a lot of detail, and you were printing on a very smooth paper, you would want at least 1440 DPI, most likely 2880 DPI, or more for advanced photographers.
An image from digital to print, for great detail should have 300 PPI. Images start at 72 in the computer because that is how we look at them on screen, and that is the size they have to be to be on the web, or less. You start with a number of pixels and if you constrain the aspect ratios in images, then the image gets smaller when the pixels per inch are increased. Photo imaging programs have components that chop up the pixels and make them smaller, halving, quartering, etc. That is how you can have the same sized image in inches, but have more pixels for a smoother look.
I have images that are greater than 500 megs, and that is not at all uncommon for photographers. Newer digital cameras shoot at 16 megs, or even more if you have the computer to handle that much, which would need about 2 gigs of ram, if not 4.
One of the tricks of all this, is to make sure your monitor is set to the highest pixel density you are going to use in your images, so that when you repair things, you see them as they will be when you print, at least in content. Color calibration is another thing. But if you set your monitor, and your printer to the same icc profile then things should be more like what you see on the screen when you print. If your monitor is only set to 800x600 you fix a photo, and print it, and there can be large messy areas, in what you thought were blacks, but really they were dark areas with a lot of detail. I have made every mistake possible, in the learning of these few basics.
By the way, an inexperienced printer will try to equate PPI and DPI and there is no real connection there. Your prints can be 720 dpi if you are using a very soft paper, like rag or watercolor, and sometimes that is better. But for smooth photographic images, tell the printer you want at least 1440 dpi. Other wise you will have droplet patterns in larger images.
