Photographic emulsion is a fine suspension of insoluble light-sensitive crystals in a colloid sol, usually consisting of gelatin.
The light-exposed crystals are reduced by the developer to black metallic silver particles that form the image.
Color films and papers have multiple layers of emulsion, made sensitive to different parts of the visible spectrum by different color sensitizers, and incorporating different dye couplers which produce superimposed yellow, magenta and cyan dye images during development.
Panchromatic black-and-white film also includes color sensitizers, but as part of a single emulsion layer.
Most modern emulsions are "washed" to remove some of the reaction byproducts (potassium nitrate and excess salts).