Emulsion is a light-sensitive material which is used to coat film and photographic paper so that images can be made to appear. The emulsion consists of light-sensitive crystals which are suspended in gelatin, creating a mixture which can be evenly applied to a base such as paper, glass, celluloid, or fabric. Technically, photo emulsion isn't a true emulsion, since an emulsion, chemically speaking, is a suspension of two or more liquids which cannot be blended, and photo emulsion is a mixture of a liquid and a solid.
For a traditional black and white photograph, the emulsion is often made with silver halide crystals. When the emulsion-coated base is exposed to light, the sensitive crystals undergo a chemical change. After the exposure, the base can be developed with chemicals which allow areas of darkness and light to appear, and then fixed with a chemical which stops the development process, freezing the levels of darkness and light at a particular stage.
In the case of film, the emulsion-coated base is used to create a negative, a reverse image of what was seen by the camera. The negative can be developed, fixed, and then used to create positive prints of the image. These positive prints, better known as photographs, are made by exposing emulsion-coated paper or other materials to a projection of the negative, or by making contact prints, in which the negative is placed directly against the photo paper and a light is briefly turned on to expose the paper. Contact prints tend to be of the highest quality, because there is no distortion from the enlarging process.
Color photography requires multiple layers of emulsion, each of which responds to light of a different wavelength. It is also possible to play with a variety of photo emulsions and filtration techniques to create things like infrared film, a type of film which responds to heat signatures, rather than light.
The materials used in photo emulsion are very sensitive, which is why film and photo paper must be stored in absolute darkness, even after exposure, until they have been developed and fixed. Usually only a brief exposure of light is necessary to expose the emulsion sufficiently for an image to appear during the developing stage. In the camera, issues like light leaks must be tightly controlled, as they can cloud the film and ruin it.
Photographers used to mix their own emulsions and carefully apply them to glass photographic plates which could be exposed, developed, fixed, and then used to develop prints on emulsion-coated glass, tin, and other materials. Most modern photographers prefer to purchase film, enlarging paper, and other supplies from companies that specialize in catering to photographers, as emulsion is difficult to handle and it can contain hazardous chemicals.