DPI and PPI are not the same! Before we can talk about scanning or printing quality, when discussing digital art, we need to understand what resolution is. Resolution is measured in DPI (dots per inch) and PPI (pixels per inch). BTW 1 inch equals 2,54cm.
Unfortunately most people confuse both terms, even though they are not interchangeable. Most people tend to talk about pixels while printing though they seem to forget that a printer is not a digital medium but a device that turns digital media in to analogue.
DPI determines the number of dots per inch when talking about printers and is independent of the resolution of the digital image. It’s important not to confuse the dots with pixels. Inkjet printers can print any given number of dots (every printer has different specs.), independent of how many pixels they reproduce while printing. To make it more clear – a printer that prints a 300 PPI digital image at 1440 DPI, uses 1440 dots per inch to print the 300 pixels per inch. The more dots per inch (and consequently per pixel) printed, produces a better tonality of the image. That’s because most home printers use 4 colors to print (industrial printers use more) and must mix the colors using the dots to produce different tones. Typical home printers print at 300 DPI.
What PPI does is set the number of pixels the image uses per inch and affectively determines the size of the printed image (and the image on screen). There are two ways of doing this – resampling and not resampling. Usually not resampling is the better idea. When not resampling changing the PPI just changes the size of the image but keeps the number of pixels in tact. If you resample when changing PPI you add or remove pixels of the image. Reducing the number of pixels isn’t a big deal but adding them usually doesn’t yield good results since the computer generates the pixels itself and the quality suffers. For web and computer use, 72 PPI is enough because that is the set resolution number of monitors.