One standard, multiple implementations

At its core, Java EE is a specification—a piece of paper, if you will. Implementations of Java EE specifications need to be developed so that application developers can actually develop server-side, enterprise Java applications against the Java EE standard. Each Java EE API has multiple implementations; the popular Hibernate Object-Relational Mapping tool, for example, is an implementation of Java EE's Java Persistence API (JPA). However, it is by no means the only JPA implementation; other JPA implementations include EclipseLink and OpenJPA. Similarly, there are multiple implementations of every single Java EE API specification.

Java EE applications are typically deployed to an application server; some popular application servers include JBoss, Websphere, Weblogic, and GlassFish. Each application server is considered to be a Java EE implementation. Application server vendors either develop their own implementation of the several Java EE API specifications or choose to include an existing implementation.

Application developers benefit from the Java EE standard by not being tied to a specific Java EE implementation. As long as an application is developed against standard Java EE APIs, it should be very portable across application server vendors.