I would dare say that less than half of the practicing hydrogeologists know the difference between hydrogeology and geohydrology. There are many other less than well-known definitions in the field. Let's go over a few so we can show off our knowledge the next chance we get.
To make it easy, the second half of the word is the science used to understand the first half of the word, i.e., a hydrogeologist uses the physical geological characteristics of the strata or rock to infer the hydrologic characteristics of the material. Clays and silty clays are usually hydrologic confining units which "restrict" vertical movement of ground water. On the other hand, geohydrology (less commonly used) is the interpretation of the geology from the hydrologic characteristics of the strata or rock, i.e., a clean sand or permeable limestone is likely to be an aquifer.