Tempera paint has a vehicle (water) & pigment just like water color, however it also has something that carries the paint and makes it thicker. The sizing and binders make it opaque.
The main difference is you can paint some light over dark, because it is slightly opaque -- however darks cannot be completely covered easily.
traditionally tempera paint was made with eggs, milk or some kind organic compounds that make it thick - "A paint consisting of pigment and glue size commonly used in the United States as poster paint is also often referred to as "tempera paint," although the binders and sizes in this paint are different from traditional tempera paint." wikipedia