Java EE applications are usually considered to be three-tiered applications, as they are distributed over three layers.
This diagram depicts a view of the three-layer architecture with the associated components. Here's a synthetic explanation:
Presentation Tier: Built with dynamic pages and application clients, this layer is able to generate the user interface of the application.
Business Tier: Also known as the middle tier, this layer contains the business logic of the application. All the business logic is centralized into this tier as opposed to client/server applications where the business logic is scattered between the frontend and the backend. The main benefit of having a centralized business tier is that the same business logic can support different types of clients.
Data Tier: This provides the information infrastructure that is vital to the business processes of an Enterprise. This includes Enterprise infrastructure systems such as ERP, mainframe transactions...