Photographic images are captured when light hits tiny particles of special salts, called silver halides, embedded in the photographic paper’s gelatin surface layer. Small portions of the salts are converted to metallic silver by the energy of the light. Before photo processing, the image will be invisible to the eye.
When the prints are developed, the salts will be converted to metallic silver particles, which through chemical reactions grow until they are large enough to see (though usually only under a magnifier). In traditional color photography, three different dye colors are formed during processing along with the development of the silver. These three colors, cyan, magenta, and yellow, can be combined in different ratios to reproduce all the colors of the originally photographed scene. However, to complete the process, the metallic silver has to be removed so that only the dyes remain.
These small dye formations are called dye clouds and, in aggregate, form the image grain. Each spot in the image below is a single dye cloud that once corresponded to light exposed and developed silver. The pattern of image grain is always random. Digitally-exposed, chromogenic photography is the only digital print process that shows image grain, so it is a defining trait that can be used in identification of these types of prints.