Technically, they are one and the same thing. But, it may differ depending on your point of view. Geology is a specialization that deals with the surface and internal structure of the Earth. Whereas Earth Science could be a collective term for all branches of Science dealing with the Earth.
Earth Science is used for Geology, Geophysics, Hydrology etc. all. Geology is only concerned about rocks, minerals, exploration etc. Apart from them, there are other things on the planet like atmosphere, life forms, oceans etc. So, Geology doesn't cover entire Earth Science.