Rivers are great collectors. As freshwater molecules flow along the landscape, gravity forces them to seek the lowest possible ground, where they gather with other water molecules. The collected water soon becomes a trickle. Trickles join together to form small creeks, which then join to form larger streams and, ultimately, rivers.
A watershed is an area of land that drains all the water in a particular area to a single point: the place where a river meets the ocean. The size of a watershed can be big or small, depending on the landscape, and some watersheds can be massive. Massive watersheds often make for humongous rivers.
Where a few creeks and streams running together end and a river begins is difficult to determine, but in order to rank rivers by their length, they've got to start somewhere and a bit of arbitrary fudging has to happen in order to measure them. However, according to the U.S. Geological Survey, "the length [of a river] may be considered to be the distance from the mouth to the most distant headwater source (irrespective of stream name) or from the mouth to the headwaters of the stream commonly identified as the source stream."
So, here they are, from the Nile to the Congo — the nine longest rivers in the world: