Raster images are comprised of small squares or pixels. The number of pixels making up an image help determine the sharpness of the image. If there are too few pixels in an image, the edges of shapes in the image will start to look jagged or grainy.
Resolution is often referred to as the number of units per inch. The unit tends to change depending on the process. For printing, resolution is often referred to as dots per inch or dpi. On a computer screen, the resolution is measure in pixels per inch or ppi.
Higher resolution images appear as continuous color to the human eye. At lower resolutions, humans begin to detect the small squares. A resolution that is ideal for printing is greater than or equal to 200 ppi at the final size. 300 ppi is very common and works extremely well.
Consider an image with the dimensions of 1800 pixels by 1200 pixels. If this image was set to be six inches wide, there would be 300 pixels per inch. If this image was scaled up to 12 inches wide, those same pixels would be expanded to reach twice as far. This would result in an image that had 150 pixels per inch. At this resolution, most people would begin to detect the small squares (pixels) on a print on paper.
Print Quality vs Image Quality
Some inkjet printers are capable of printing 1440 dpi. This can provide incredible crispness to the final print of an image and the illusion of continuous color. This is the print quality. Print quality will also be affected by the surface being printed on. A high quality photo paper will produce a finer print than a piece of fabric.
Image quality is largely affected by the resolution of the image itself. It is separate from the print quality. If a low resolution image (72 ppi for example) is printed by a printer using 1440 dpi, the quality of the print may be amazing, but it will be an amazing print of a poor quality image. The low resolution image will have pixilated edges. The higher print quality will only create very clean, sharp pixilated edges.
DPI vs. PPI
Often used interchangeably, dpi and ppi very similar, but they are not exactly the same.
The abbreviation dpi stands for "dots per inch", and is used more in reference to printing and dots of ink.
The term ppi refers to "pixels per inch" and is used when speaking of the resolution of a digital image. Both terms are a way of measuring resolution and they do so by counting sample units per inch.