Nowadays, there are several ways to “render,” or image and display, output— printers, monitors, tablets, and smartphones (and I’m sure I’ve left a few out). In one way or another, they all gauge output density with some sort of resolution measurement, such as dots per inch(dpi) or pixels per inch (ppi). Typically, though, we use dpi when referring toprinted resolution, and when we use ppi, we’re talking about image, or display, resolution.
It’s also important to point out that laser-class printers turn out entirely different size and shaped dots than do their inkjet counterparts. For example, a good, appropriately configured laser printer’s output is so clean, in terms of screen frequency and halftones, that you can (theoretically) use it as camera-ready art, i.e.color separations, for reproduction on a printing press. (However, most designers would use the laser output for proofing, and then go one step further and output the color separations to film for printing on a high-end, high-resolution imaging, or typesetting, machine.)
Read the entire article at About.com.