What do dpi and ppi mean?
  Dpi and ppi are standard ways of measuring how the data in an image file is organized. They tell you the resolution of scanners, printers and photo files (this is entirely different than the optical resolution of a photo based on the film and lens). Dpi means dots per inch and refers to the number of dots (or pixels) of a photo that fit into an inch. Ppi refers to the number of pixels per inch – the two terms are often used interchangeably. Dpi is more common. Both dpi and ppi can change to fit different needs (different printing devices need different dpi’s) – scanners can be set to different resolutions and photo files can have their resolution changed for different purposes. If you compare two photos of equal size but at different dpi, the one with more dots in an inch will allow more detail to be displayed (it will have a higher resolution). Dpi affects the way a photo is captured by a scanner and how it looks when printed out.

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