The key to understanding resolution is to know each term and how it is used. There are four main types of measurements of resolution.
1. SPI (samples per inch) is the terminology used to describe how a scanner measures resolution. This is the number of individual samples taken in an area of one linear inch. The more samples the scanner takes, the more detailed the scan will be and closer to the original image being scanned.
2. DPI (dots per inch) is used by printers and refers to the number of dots of ink used by the printer to reproduce the image. The higher the number of dots per inch, the sharper the image. DPI is the resolution of the printer or other digital output device.
3. PPI (pixels per inch) refers to the number of pixels displayed on a computer monitor, the display resolution. It also refers to the image resolution and the reproduction size. For example, an image size of 1600 x 2500 pixels at 300 ppi will be 8 by 5.3 inches when printed. A higher PPI gives greater detail and sharpness.
4. LPI (lines per inch) is how printers print a picture using halftone screening. This refers to how close the lines are in a halftone grid. Higher LPI means more detail and sharpness.
Some people use many of these terms interchangeably, but they do not refer to the same measurements or reproduction sizes. Books, tutorials and manufacturers use many of these terms to encompass a variety of output uses. Consider the context of how the image is going to be reproduced to help you figure out which of the above terms actually applies. Understanding these terms will help to understand the different digital devices from camera resolution to output printing resolutions.