Wow! I just found this thread and it IS a can of worms... Here goes my feeble attempt at explaining DPI and PPI. This is going to be terribly
PPI has to do with the ACTUAL resolution of your digital file. DPI is the print resolution. The first thing to remember is that they are not interchangeable - apples and oranges... PPI
When you were in school in art class, did you ever have to make a pointalist drawing using only dots you made with your pencil? OK, then think of that as PPI. Now imagine you made a drawing like that not on paper, but on a flat sheet of rubber. Now stretch it... No matter how you stretch or contract that rubber, the drawing you made will never have any more or less pencil dots on it. It will only appear
bigger or smaller. The more dots you put on it, the bigger you could stretch it before it started looking distorted. DPI
How many of you have ever done a silk screen print? Ok, good. Same rubber photo above. Same number of pencil dots. But your rubber now has some really neat properties that are completely against the laws of physics. It can liquify on command (think Odo on Star Trek - DS9). You can make a screen print from that drawing by simply laying the rubber on the screen, telling it to liquify, and then squishing it through the screen onto your T-shirt. However, Odo cannot reconnect himself once he has been squished through the screen. However many dots are in that screen are the number of dots he must remain in after being squished. The screen is DPI. Still no change in the original dots on the rubber. But the number of dots the rubber has been squished into on your shirt can be different depending on whether you have squished your Odo-ized print through a screen with holes the size of chain link, or if the holes are more like the ones in your screen door...
Gawd I hope that makes sense...!