Engineered wood floors are built having multiple thin ply layers (see picture below) that are glued together. The inner cores are generally either a hardwood and/or soft plywood type of material, which incorporates the tongue and groove system. The top thicker hardwood veneer wear layer is glued on top surface of the core and is available in almost any hardwoods.
Engineered wood floors come in a wide variety of hardwood species. Some brands of engineered flooring have a thin wear layer that can only be re-coated and cannot be sanded and refinished new again once they get worn. They have an average lifespan of between 30 - 40 years depending on traffic.