This is where all business rules are applied as data passes from the PresentationLayer (UI), through the business Layer to the data Access Layer, and back again.
With a thin client, you can maximize the one resource you have complete control over, the server, to do the brunt of the work and dish out a minimal set of data and presentationlayer information.
Business rules are centralized into the business logic layer that serves as an intermediary for data exchange between the presentationlayer and the data access layer.