Fom the Sun site.
This tier represents all device or system clients accessing the system or the application. A client can be a web browser, a Java or other application, a Java applet, a WAP phone, a network application, or some device introduced in the future.
This tier encapsulates all presentation logic required to service the clients that access the system. The presentation tier intercepts the client requests, provides single sign-on, session management and accesses business services, constructs the response, and delivers the response to the client. Servlets and JSPs reside in this tier.
This tier provides the business services required by the application clients. The tier contains the business data and business logic. All business processing for the application is centralized into this tier. The enterprise bean components are the usual choice for implementing the business objects in the business tier.
This tier is responsible for communicating with external resources and systems, such as data stores and legacy applications. The business tier is coupled with the integration tier whenever the business objects require data or services that reside in the resource tier. The components in this tier can use JDBC, J2EE connector technology, or some proprietary middleware to work with the resource tier.
This is the tier that contains the business data and external resources such as mainframes and legacy systems and business-to-business (B2B) systems, and services such as credit card authorization.