watershed is the land which "sheds water," or drains, into a particular water body. The watershed is also an ecological system based on water, supporting all the life in that area

Water that falls on the region (as rain, snow, etc.) drains downhill, so hills, ridges and other high points define the boundaries of a watershed. The water flows over the surface of the land and underground (an aquifer) that converge into streams and rivers. As you move downhill and this network of streams and rivers converge and progressively grow larger, eventually reaching the ocean 

  

Watersheds can be large or small. Every stream, tributary, or river has an associated watershed, .and small watersheds join to become larger watersheds . Watersheds can also be called basins and drainages. Here is an example of what a watershed looks like .