The J2EE standard defines a rich set of components with which complex distributed applications can be built, such as servlets, Java server pages, and Enterprise Javabeans. Each component has a specific and well-defined set of roles, which they can play in a complex distributed system.
J2EE enterprise service APIs interface between specific vendor drivers/adapters and J2EE components. They expose standard API calls over vendor-specific service implementations, so that the application components are independent of vendor-dependent codes.
Services available in J2EE include database connectivity, e-mailing, distributed directory access, XML parsing/binding, and CORBA communications.
To understand the technical aspects of J2EE platform, please read my earlier article "Inside J2EE." To know different types of client requests that can be serviced by J2EE components, please explore my next article, "J2EE Clients."