Dots per Inch (DPI) and Pixels per Inch (PPI)
Probably the most confusing and often misused terms in digital photography are dpi and ppi. Manufacturers often don't use these terms as accurately as they should, and even professionals get them confused.
ppi: Pixels per inch means just what it says, the number of actual pixels in one inch. For example, at 300 ppi a 5″ × 7″ inch print would be 1500 pixels by 2100 pixels.
dpi: Dots per inch is really a printing term but is often used to mean ppi, which it is not. The resolution of a printer, for example, is listed at dpi, but in this case each pixel may be printed as several dots.
To get the best possible print, you want the ppi to be in synch with the dpi output of the printer. However, you may need to read the printer manual and/or talk to a knowledgeable person at the print shop.