Enterprise JavaBeans Technology

server-side component architecture for Java Platform, Enterprise Edition (Java EE)
enables rapid and simplified development of distributed, transactional, secure and portable applications based on Java technology
JSR 220 FR: EJB 3.0 Documentation/Specification
- defines EJB API for development of enterprise applications
- defines dependency injection facility
- defines query language for Java Persistence that is an extension to EJB QL
- introduces Java language metadata annotations to be used as an alternative to deployment descriptors
- defines metadata annotations and XML deployment descriptor elements for the object/relational mapping of persistent entities
- provides an interceptor facility for session beans and message-driven beans

developer uses enterprise bean class as the primary programming artifact
developer defines enterprise bean class and annotates it using Java metadata annotations
bean type of enterprise bean class must be specified by means of metadata annotations; otherwise deployment descriptor elements may be used
session beans and message-driven beans require a business interface
business interface of an enterprise bean is a plain Java interface
business interface of a message-driven bean is typically defined by the messaging type used (e.g. javax.jms.MessageListener in the case of JMS)
The bean class may implement its business interface(s). A bean class may have more than one business interface
While it is expected that the bean class will typically implement its business interface(s), if the bean class uses annotations on the bean class or the deployment descriptor to designate its business interface(s), it is not required that the bean class also be specified as implementing the interface(s).

Rules which apply when defining a bean class:

if bean class implements a single interface, that interface is assumed to be the local business interface of the bean unless the interface is designated as a remote business interface by use of the Remote annotation on the bean class or interface or by means of the deployment descriptor
if bean class has more than once business interface (other than java.io.Serializable; java.io.Externalizable; or any of the interfaces defined by javax.ejb package), that interface must be explicitly designated as a business interface of the bean by means of Local or Remote annotation on the bean class or interface or by means of the deployment descriptor
same business interface cannot be both a local and a remote business interface of the bean (class). It is also an error if @Local and/or @Remote is specified both on the bean class and on the referenced interface and the values differ.
business interface must not extend javax.ejb.EJBObject or javax.ejb.EJBLocalObject

Interceptors

interceptors may be defined for session beans and message-driven beans
interceptor is a method to intercept a business method invocation or a lifecycle callback event
interceptor method may be defined on bean class or an interceptor class associated with the bean
interceptor class is a class whose methods are invoked in response to business method invocation and/or lifecycle events on the bass class

Notes

JSR-175 specification of J2SE 5.0 allows developers to annotate program elements in Java programming language source files to control the behavior and deployment of an application.
Why do you need annotations/metadata? -
Metadata annotations are a key element in the simplification of the development of EJB applications. They are used by the developer to specify expected requirements on container behavior, to the request injection of services and resources, and to specify object/relational mappings. They might be used as an alternative to the deployment descriptors. However, it is not required that metadata annotations be used in EJB 3.0 application. Developers who prefer the use of a deployment descriptor as an alternative to metadata annotations may define one for this purpose. It is possible for the application developer to combine the use of metadata annotations and deployment descriptors in the design of an application. When such a combination is used, the rules for the use of deployment descriptors as an overriding mechanism apply
The metadata annotations to specify that a bean implements a web service and how the web service is exposed to clients are defined by JSR-181, “Web Services Metadata for the Java Platform.”