DPI stands for Dots Per Inch and is the resolution printing machines output at. This is a measure of how fine the particular printer’s output can be. The higher the number the more dots the printer can squeeze into an inch resulting in a clearer print quality.
PPI - Pixels Per Inch is basically the resolution of your monitor or screen of the device you are using to design your artwork. A pixel is the smallest point that the screen will produce, a mix of primary colours red, green and blue that combine to give the required hue.
When designing artwork your concern should be ppi, the majority of images taken from the web will be 72ppi, however, this quality resolution will not be high enough to give you a good printed result. Both dpi and ppi produce a better image at higher numbers, go as high as possible. We recommend a minimum of 350ppi up to 600ppi for optimum quality.
Please note that opening a low-resolution 72ppi image in Photoshop and simply increasing it to 350ppi will not increase the quality of the image, you need to open the original (native) file then increase the resolution to your desired ppi.
This example shows the difference in the printed quality of a 72ppi image (low screen resolution) vs 350ppi high-resolution printable image.
A 72ppi image may look perfectly fine on a website but the resolution is far too low to achieve a high-quality result in a printing process.