Digital ProductionRevised
PPI
Pixels per inch — a measure of an image's own density, independent of the device it's printed or displayed on.
In practice
PPI describes how many of an image's pixels fill each inch when it is shown or printed at a given size. The pixel count is fixed, so PPI changes with the size chosen: a photo 3000 pixels wide is 300 ppi at 10 inches wide but only 100 ppi at 30 inches. Printers commonly want around 300 ppi at final size for material seen up close, while screens care mostly about total pixel dimensions.
Often confused with DPI
PPI is the pixel density of the image itself; DPI is the number of ink dots a printing device lays down per inch.
Try this
Take one photo's pixel width and calculate its PPI if printed as a postcard, a magazine page and a poster.