The layers of a Business Application are the Presentation layer, Application Layer & the Database Layer.
There are several ways to configure these layers in a client/server environment. In a three-tier configuration, Individual servers are used for each layer.
ABAP Programs run at the application layer. The Programs work with data from the database layer. The database layer is where the data is managed. Data includes master, transactional and metadata. The Presentation layer contains the user interface.