The area of land that is drained by a particular river and its tributaries is called either a river basin or drainage basin. This area is also commonly referred to as a watershed, which can cover either a small or large area of land depending on where it begins and ends.
All water that is contained within a specific river basin flows to the same endpoint, which can be a sea, ocean lake or even another, larger river. The Mississippi River watershed is an example of a large watershed, as it includes a number of smaller rivers that flow into it, all of which eventually flow out into the Gulf of Mexico. All of these smaller rivers also contain their own watershed, as each is made up of different sources and tributaries that flow into the same endpoint at the Mississippi. Most large watersheds are made up of many smaller watersheds, although some small ones can be self-contained if they flow directly into the sea or ocean.
The source of a river or the sources of its tributaries are the beginning of the watershed. These sources can include springs as well as runoff from snow and glaciers. The end of the watershed is always the mouth of the river, which is where it flows into the larger body of water.