There are two main types of stucco systems that are used and these are hard coat systems and EIFS systems which can look very similar when finished but are in fact, very different.
Hard Coat Stucco Systems - These are cement based stucco systems, which are why they are referred to as "hard coat" systems and have a base coat of cement that is applied before the finish coat is applied (in most cases).
These can range in depth, depending on a few different factors like which stucco system you are using (three coat or one coat/two coat system), what substrate you are going over, etc. The general thickness for base coats can be from 3/8" - 7/8" typically.
The base coat is usually comprised of cement (typically Portland cement), sand and water and gets extremely hard, if properly cured.
This is referred to as the base coat that will make a suitable foundation for the finish coat to be applied. The base coat is also where the "hard coat" stucco system gets it's name.
EIFS Stucco Systems - An EIFS system is actually an acronym for Exterior Insulation and Finish System and does not rely on a hard base coat but instead is thinner layers of modified materials that make up the EIFS cladding.
EIFS is usually a little different and is known to have better water barriers and insulation properties than traditional hard coat stucco systems and most EIFS assemblies are designed to keep water on the outside of the building.
A hard coat stucco system will absorb water and it will leave the system through evaporation and/or through the weep screed.
The best example would be to look at some of the most common EIFS claddings out there to get a better idea of how they work.
Different manufacturers have their own special systems but the "basics" are essentially the same, having little differences between the many systems.