Link to the FAQ first... don't worry about/change DPI here basically.
http://www.oprworkshop.org/forum/index.php?topic=61.0Rule of thumb = digital images and desktop displays are measured in pixels, prints are measured in inches.
On your screen, all you need to be concerned with is the number of pixels. For instance, let's say I have an image that is (width x height) 3000x2400 pixels. Multiply that and you get 7,200,200 pixels or roughly 7.2 megapixels. Let's say it's 3000x2400 and 72 DPI (or PPI if you want to be more correct). 3000 / 72 = 41.6666. That means at 72 PPI/DPI it would be 41.6 inches wide. Similarly, 3000 / 300 = 10 inches wide. Whether the file is 300 or 72 DPI/PPI it's still 3000x2400 pixels.
DPI and PPI have 2 things in common - "PER INCH" and we only measure prints in inches. PPI and DPI are really meaningless when it comes to what's on your monitor... it's all about pixel count and the more pixels the better usually. 640x480, 800x600, 1600x1200, those are the kind of numbers that matter on a monitor, they are measurements in pixels and the letter 'p' is maybe never noted/added when they are used - as opposed to dealing with physical objects measured in inches or centimeters, you better darn well note/add the 'in' or 'cm' to your numbers in those cases!!! DPI/PPI is more important if you are working with something that you are going to print yourself. If someone else is going to print it, just give them the biggest file (or the original size) you have unless they specify something different.
D = dots and P = pixels.
Pixels in PPI are part of the electronic imaging device that you are using to capture an image like a scanner, and the PPI setting refers to what will be output when you scan... scan an 8x10 @ 300 PPI and you get a file that is 2400x3000 pixels - it's simple math.
Dots in DPI refers (more or less) to the drops of ink put onto paper by a printer. I don't care how poorly labelled some programs and books (I aint
even gonna talk about websites) can be regarding DPI/PPI that's just the way it is.

More info and maybe better explained at these links:
http://www.rideau-info.com/photos/mythdpi.htmlhttp://en.wikipedia.org/wiki/Dots_per_inchhttp://en.wikipedia.org/wiki/Pixels_per_inch