The quality of a digital image, whether printed or displayed on a screen, depends in part on the number of pixels used to create the image (sometimes referred to as resolution). The maximum number that you can capture depends on how many photo sites there are on the image sensor used to capture the image. (However, some cameras add additional pixels to artificially inflate the size of the image. You can do the same thing in an image-editing program. In most cases this upsizing only makes the image larger without making it better.)
More pixels add detail and sharpen edges. If you enlarge any digital image enough, the pixels will begin to show-an effect called pixelization. This is not unlike traditional silver-based prints where grain begins to show when prints are enlarged past a certain point. The more pixels there are in an image, the more it can be enlarged before pixelization occurs.
The size of a photograph is specified in one of two ways-by its dimensions in pixels or by the total number of pixels it contains. For example, the same image can be said to have 1800 x 1600 pixels (where "x" is pronounced "by" as in "1800 by 1600"), or to contain 2.88-million pixels (1800 multiplied by 1600).