Let us first clear up a common confusion between ppi and dpi. Digital images are composed of samples that are displayed as pixels, so ppi (pixels per inch) refers to the number of pixels in an image. This is called display resolution.
Dpi (dots per inch) is not the resolution of scanned or on screen images, it is the printer’s resolution. It refers to the number of ink dots a printer uses to print a text or image. The more dots per linear inch, the more detail your image will show. Keep in mind that a dot is not necessarily the same as one drop of ink. Hewlett Packard's PhotoRetIII technology accurately places up to 29 tiny ink droplets in a single dot, whereas some Canon printers use a print head with 1,472 nozzles and ink droplets as small as 2 picolitres.
Required image resolutions (ppi) can be calculated by multiplying print size by print resolution. An 8” x 10” image in a magazine at 300 dpi requires a resolution of 2400 x 3000. Newspaper images are 180 dpi while resolution for home printers will range from 250 to 300 dpi for colour laser printers, to 2400 dpi for professional photo printers. Printing at higher dots per inch than the printer's optimal image resolution is useless since it will not result in better image quality and only makes file sizes larger than necessary.