These terms are not "GIS" terms, they've been around for a while. I�??ve always understood that watershed and drainage basins are different terms for the same thing, an area where water drains to the same place. The term catchment is typically the same thing but the word refers to the amount of water caught in a given area of a basin/watershed.
�??Catchments and watersheds are land areas that drain to a Hydro Network. The determination of their boundaries is necessary when modeling a hydrologic system. Drainage boundaries are used in water availability studies, water quality projects, flood forecasting programs, as well as many other engineering and public policy applications. Accurate drainage boundaries are essential for accurate modeling studies. Through the ArcGIS Hydro Data Model and associated ArcInfo functionality, watershed and catchment boundaries may be determined accurately and repeatedly in an automated fashion...
�?�A catchment is defined as the land area that contributes runoff to a given Hydro Edge. In the ArcGIS Hydro Data Model, catchments are intimately associated with the Hydro Edge about which they are formed, and as such are referred to as Edge Catchments. Any raindrop falling on an Edge Catchment has a unique path to a Hydro Edge and thus to being routed down through the Hydro Network. Edge Catchments are polygon features in the data model�?��?�
