What is the difference between PPI and DPI?

PPI (pixels per inch) is a unit of resolution of the device. Often this unit is used for display purposes on screens. For color photos they commonly use the 300 PPI standard. (Although often the term DPI instead of PPI is used since they use the same value)

DPI is the number of ink dots per linear inch. DPI is a unit which is intended for print and not imagery.

1 DPI equals one PPI.