We use images two ways: on paper, or on video screens.

Image size is measured on paper in inches or centimeters, while image size for a screen is measured in pixels. Images on paper are produced with lots of small dots of ink. Images on a screen are produced with lots of points of colored light called pixels.

Printer ink dots and image pixels are very different concepts, but both use the term dpi in their own way. Dpi is an abbreviation for dots per inch. Ppi stands for pixels per inch, a more correct term for screen resolution. However dpi has always been commonly used to refer to image resolution, or pixels per inch. The term is fully interchangeable with ppi.

Use the term you like, but understand it both ways. If you are dealing with images output to a screen, in pixels, dpi means "pixels per inch". If you are examining printer ratings or referring to printing photos, dpi means "ink dots per inch".

If you have a 300 dpi image, both terms mean it will print at 300 pixels per inch (pixel spacing on paper), so 300 pixels will cover one inch.