What makes a Java class an enterprise bean?
An enterprise bean is composed of many parts, not just a single class. Essentially, an enterprise bean is constructed with a bean class, remote interface, home interface and deployment descriptor. These constituents are discussed below.
A bean class is the implementation class of the bean that defines its business, persistence, and passivation logic. The bean class implements either the javax.ejb.EntityBean or javax.ejb.SessionBean interface and runs inside the EJB container. Instances of the bean class service client request indirectly; instances of the bean class are not visible to the client.
The remote interface defines the business methods that will be visible to the client's that use the enterprise bean. The remote interface extends the javax.ejb.EJBObject interface and is implemented by a remote (distributed object) reference. Client applications interact with the enterprise bean through its remote interface.
The home interface defines the create, delete (remove), and query methods for an enterprise bean type. The home interface extends the javax.ejb.EJBHome interface and is implemented by a remote (distributed object) reference. The client application will use the home interface to create beans, find existing beans, and remove specific beans.
The deployment descriptor is used to describe the enterprise bean's runtime behavior to the container. Among other things the deployment descriptor allows the transaction, persistence, and authorization security behavior of a bean to be defined using declarative attributes. This greatly simplifies the programming model when developing beans.
An enterprise bean represents the sum of all these parts (remote, home, bean class, and deployment descriptor) as one component. An enterprise bean is not an enterprise bean if any one of these parts is missing. A change to anyone of these parts -- changing even one attribute in the deployment descriptor for example -- creates an entirely new enterprise bean.