Image noise can be defined as appearance of undesired traces and variations in the brightness or color of your image. These random imperfections make your image grainy, thus affecting its quality.
There are two types of noise: shot noise and digital noise. While shot noise is caused by random photons in the photographed scene, digital noise is due to your camera sensor. Although they come from different sources, both types of noise are very similar, and almost indistinguishable.
Difference in noise level. The left image is “noisy”: it is grainy and render imperfections.