A watershed is the area of land where water drains downhill into a single body of water.

The Watershed Concept - A Flash Animation (Michigan Tech University)

The U.S. Geological Survey has defined roughly equal-sized watersheds for the United States at various scales, and a Hydrologic Unit Code (HUC) has been assigned to each watershed. 

Just as a house is assigned a numeric zip code, watersheds are assigned a numeric locator, called a Hydrologic Unit Code (HUC).  A HUC can have anywhere from 2 to 12 digits.  The more digits, the smaller the land area of the watershed.  In this website, you will find information for 8-digit Hydrologic Unit Codes.  There are 57 of these 8-digit watersheds in Arkansas.

Arkansas Watersheds - An Overview (University of Arkansas Cooperative Extension)

Major basins, like the Mississippi River Basin, are sub-divided into smaller basins, like the Arkansas River Basin and the Red River Basin, which are further sub-divided into smaller and smaller watersheds.  The smaller, more local the watershed my neighbors and I decide to focus our efforts on -- the more likely we can generate measurable results.  And results get people excited!

Find Your Watershed  (Arkansas Watershed Information System)