Cutter,
Good question , and I didn't know , so this is what I found , hope it helps
DPI vs PPI
Technically DPI means paper printer Dots Per Inch while PPI means digital Pixels Per Inch. But in many instances the term DPI is used when in fact PPI is what is really meant. Even in these articles there is crossover of terms, since many software programs continue to use the legacy term DPI to mean PPI. So, best to read the terms in context. When DPI is mentioned are they really talking about printer dots per inch or are they in fact talking about how many pixels per inch a digital photo is going to print out at? This too is where many print shops, graphic artists and magazines have it wrong when they are talking about required digital photo resolution, they mistakenly think the DPI setting in a photo has something to do with this, when in fact what they want is a certain PPI (see What Print Shops Really Want).
Some programs such as Adobe Photoshop are now calling the setting of what used to be shown as DPI as pixels/inch or pixels/cm. It's a bit of one step forward, two steps back since they still have it sitting in a section called "Resolution" leading people to mistakenly think that it has something to do with the digital resolution of the photo. Some camera manufacturers such as Canon have it right, they properly refer to digital resolution as the pixel dimensions of the photo.
To be clear, neither the DPI or PPI setting in a digital photo changes the digital quality of that photo. The resolution of a digital photo is its pixel dimensions
If that hasn't answered the question here is the page from whence it came
http://www.rideau-info.com/photos/mythdpi.html