An image’s resolution is determined by the number of individual addressable points making up the photo, whether it is the number of dots that comprise a printed image, or the number of pixels contained in a screen image.
Depending on several factors, typically the more dots that are used to create an image, the more detail the image displays, resulting in sharper, better-looking scans and prints’providing you start with quality images to begin with, of course.
When, for example, you use bitmap graphics, whatever resolution you choose, information for each pixel or printer dot needs to be stored. The higher the resolution, the more information needs to be stored for any image of any size.
The only place this does not apply is when you’re using vector graphics (which isn’t often in most scan-or print situations), as the information about resolution is relevant only when the image is printed, or exported as a bitmap.
Read the entire article at About.com.