Assembly: System.Web (in System.Web.dll)
A Web Parts page always has a single instance of a WebPartManager control. The WebPartManager control tracks and manages all the other Web Parts controls on a page, including all zones that derive from the base WebZone class, and all part controls that derive from the base Part class.
The property exists so that every WebPart control can be associated with its WebPartManager control. The WebPartManager control, in turn, maintains a collection of all the WebPart controls on a page in its WebParts collection.