EJB is a complex system designed to address the bulk of enterprise-level requirements discussed earlier. It defines a contract between several roles in Java software provision. At runtime, these roles fall into two categories, and much of the power of EJB is found on the interface between them. Application software takes the form of components — "beans" or "EJBs" — each of which implements some business logic (session beans) or persistence (entity beans) function. These components are loaded into a container which manages them: controls their lifecycle, provides remote access to them, monitors client access for purposes of security and transaction control, and more.

The EJB value proposition is that by using an EJB container/server, application developers can offload the tedious and repetitive chores of security and transactionality, and more rapidly build components by focusing on application logic and features. Scalability and availability issues are also addressed by the EJB container, as it can support pooling of component instances to support large volumes of client requests (for session beans) and large volumes of persistent data (for entity beans).