Frankly, you can yse as many types of structures as you want. Some categorize by file types, some by tags, and some by the relations. However, I found the following folder structure to be very efficient.
I have come across many blogs and studies their folder structures and what I found best for me was the following one. This is a lightly edited version of what you will get at : http://www.glenstevens.ca/unity3d-best-practices/