The following chemicals make up basic paper developers. Differing ratios of these give the developers their characteristics.
Each has a specific job within the action of the developer. I've included a brief description of each.

Characteristics of Chemicals:

Metol: Low contrast reducer (developing agent). Non staining, clean working, resistant to cold. Non-soluble in sodium solutions. Fast development times in carbonate solutions. Fine grain, won't block highlights (in negatives) when used alone.

Hydroquinone: High contrast reducer (developing agent). Non staining, clean working, soluble in warm water. Extremely sensitive to cold and bromide. Produces high speed, high contrast developer in strong alkaline solutions.

Sodium Sulphite: Preserving agent. Because of the reducer's hunger for oxygen when mixed with alkaline, the developer needs a preservative to slow down the oxidization process.

Sodium Carbonate: Activator. An alkaline solution which, when mixed into the developer turns into an alkali salt. This salt acts to transform the silver salts on the film or paper in metallic silver. The alkali in the solution also acts to swell the gelatin emulsion which aids the penetration of the developing solution.

Potassium Bromide: The restrainer. Controls the development process. An increase in bromide solution will slow the development times.