DPI is dots per inch, and is often used to describe the (max) resolution a printer can offer, but it is also applicable for scanners. Contrary to PPI, the DPI for a printer or a scanner is not fixed but can vary subject to the print mode. So you can print and scan at different DPI levels, subject to your needs. The higher the resolution required, the longer the scan or print will take, so when you don’t nee the high resolution, it is nice to be able to prioritise speed over resolution.
When printing, the DPI needs to be significantly higher than the PPI for a screen, and this is because the printer builds a colour combining different base colours, and hence more base colour dots in combination are needed to make up for a similar dot on the screen. So when you read the spec sheet for a good colour printer, the DPI is often much higher than the PPI for a screen.
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