Using Map Viewer (or Map Viewer Classic) of ArcGIS Online or ArcGIS Enterprise, you create a Web Map.
With ArcGIS Web AppBuilder of ArcGIS Online/ArcGIS Enterprise/Developer Edition, you create a Web App. With a Web App, you can select from different configurable templates, add many custom and ready-to-use widgets.
There are many other App Builders that one can use. Here is a list.
To add to this a bit, you typically create a Web Map to be an important component of a Web App. The "App" is all the widgets and extra bits that help narrow the focus of the underlying web map.