Image resolution refers to the amount of finite detail a photo has. This terminology is used for both film and digital photography. Resolution can be measured in the number of light and dark lines per millimeter or the amount of pixels used to make a photo visible and will often be written on a film camera as LP/mm or in megapixels on a digital camera. An image is considered low-resolution if it uses a small number of pixels to render the image, the dots per inch number is low, size is small or it has a low spatial quality.
A single pixel is a small square color dot. Millions of pixels can be formed together like a mini-mosaic to make a high quality image. Low-resolution images use fewer pixels than high quality images, and it shows. In a low resolution image it is much easier to pick out the square pixels because rather than having several hundred pixels of varying color shades that make one color, to the naked eye you will have several hundred pixels of one color that will render as a large square block from far away. This will make the image particularly blocky and there won't be any smooth edges.
Dots Per Inch
Scanners and computers use "dpi," dots per inch, to render the resolution of images on a computer and on the Internet. A low resolution of 72dpi is actually fine to use on the Internet and on your home computer. But if you scan an image on a low dpi and then print it, you will see the difference. While most photo editors allow you to increase the dpi after an image has already been scanned, this is actually just stretching out the photo. This will cause the photo to have a blurry appearance when printed. To prevent this, scan all images with at least 300dpi, or when creating digital images on a computer start your blank canvas at 300dpi.
A low-resolution image will be much smaller than a high-resolution image. If you render it on a screen, it will appear tiny, maybe around an inch or two high and across. If you zoom in on the image you will see that the image becomes blurrier or pixilated the closer you get. While from far away the image looks small, but clear, you cannot increase the size of the image or you will lose all photo quality.
Some images may have a high number of pixels and a high dpi, but may still appear blurry at a large size. These images have a low spatial resolution. The camera, scanner, or the computer that you are using to render an image, and not necessarily the image itself, causes low spatial resolution. Spatial resolution is measured in ppi, or pixels per inch. These pixels are independent features that can be detected by the machine. If these pixels are not picked up properly, the image will still render but it will be blurry over the edges as if some information is missing.