Dpi is the printer resolution, ppi is the file resolution.
Ppi is what you alter to change the print size and should always be equal to or less than the dpi. You then send the file to the printer which will print it at the print size you set at the dpi resolution, which for most commercial print houses is fixed at 300dpi.
Contrary to what you might guess SMALL pictures require a higher dpi and larger prints less.
Chris
