What's the difference between demolition and excavation?

Demolition is the act of tearing down a structure, like a house or building. It has its own name because the process itself is a skilled trade. Demolition crews must know how to safely and effectively bulldoze or tear a house down, how to handle different materials and occasionally even how to salvage certain materials from the house. 

Excavation is the use of excavators to perform manual work like digging, trenching and tunneling. It also often involves surviving the ground, removing topsoil and clearing rocky surfaces.