A watershed is the area of land where all of the water that is under it or drains off of it goes into the same place.
John Wesley Powell, scientist geographer, put it best when he said that a watershed is:
"that area of land, a bounded hydrologic system, within which all living things are inextricably linked by their common water course and where, as humans settled, simple logic demanded that they become part of a community."
Watersheds come in all shapes and sizes. They cross county, state, and national boundaries. In the continental US, there are 2,110 watersheds.