The application server maintains control and provides services through an interface or framework known as a container. There are five defined container types in the J2EE specification. Three of these are server-side containers:

• The server itself, which provides the J2EE runtime environment and the other two containers.
• An EJB container to manage EJB components.
• A Web container to manage servlets and JavaServer Pages.

     The other two container types are client-side:

• An application container for standalone GUIs, console, and batch-type programs — the familiar Java applications started with the java command.
• An applet container, meaning a browser, usually with the Java Plug-in.

     There are three kinds of defined components:

• Client components, which correlate to the client containers.
• Web components — servlets and JSPs.
• EJB components.

     In the server-side J2EE context, a component is a unit of functionality that is assembled, along with any required resources, into a J2EE application. Note that a "J2EE application" may be a single EJB. The server-side components can communicate with each other inside the J2EE environment (intra-VM on a single machine) or in a networked, multi-machine, distributed environment. The client-side containers communicate with the J2EE container and server-side components through a client JAR generated at deployment time. Client-side containers can be on the same machine or, more typically, somewhere on the network. Also, components can be clients of other EJB components — that is, they can call on the services of other beans.

     While any or all of the containers and components may be on the same or multiple machines — remember, J2EE is a distributed model — conceptually, the architecture is almost always three tier. These tiers are client, business logic, and database or Enterprise Information Systems (EIS).