Although many use them interchangeably DPI (dots per inch) is *NOT* the same as PPI (pixels per inch). Even when your printer Asks you a photo in 300 DPI he actually means he needs a 300 *PPI* image. He is probably aware of the confusion between the two and just plays along for communications sake. Printers use a variable number of small ink dots per to come to 1 colored digital pixel. The trouble is that different print processes use different number of ik dots per pixel. Most of the time the request comes from a printer that uses a 4 colour offset printing technique. And in those cases he actually needs a 150 DPI. 150 DPI roughly converts to a 300 *PPI* image. So the printer does not care that you do not know the difference, actually he bets on it, and just asks you the 300 DPI file. When you create a new image in Photoshop check the dropdown menu under 'resolution' it also says 'pixels/inch' and not 'dots/inch'.



Even after years of working with Photoshop we assumed it said dots and not pixels because that was what clients were asking. So Although it's bad to add to this confusion by using DPI/PPI as if it was one and the same for many people it would be just as confusing using only PPI on imageafter. We recommend you read the following links to pages written by Andrew Dacey and Design215 about DPI vs PPI. They explain the difference far better that we could:

DPI and PPI Explained
Photography Printing Guide

Wikipedia on DPI
Wikipedia on PPI