Film is, very simply, a light-sensitive emulsion on a plastic base. An easy way to think of film is to compare it with bread and butter. Think of the bread as the base, the butter as the emulsion. As butter gives your bread flavor, emulsion gives your film an image-recording ability.
Emulsion is a gelatin made of millions of light-sensitive silver halide crystals.
In fact, if you could examine the emulsion before and after exposure to light, you would see no difference. Although the image is there, it's latent. It has to be processed before it becomes a picture that you can see. When you take a picture here's what happens. The shutter opened and closed for only a fraction of a second.