Generally speaking, there are two main chemicals used in b&w film, and a couple of auxiallry chemicals. The primary chemicals are developer, which causes the silver iodide of the film or paper emulsion to darken if it has been exposed to light, and fixer, which removes the undeveloped silver iodide from the emulsion. A chemical used to completely stop the development process is called stop bath, and is used between the developer stage and the fixer stage in both b&w film & paper processing.