What is Java EE container

For Java SE (or J2SE) applications JVM provides runtime environment, but the moment you switch to Java EE (or earlier known J2EE), runtime environment is provided by web or application servers.

JVM executes/runs Java applications and also provides some of the runtime services like lifecycle management for objects and API support etc. Life is not so rosy in the distributed environment and that's why we have servers for specific jobs. These servers basically fall into two categories, application servers like JBoss(Wildfly) and Glassfish and web servers like Tomcat and Jetty. These servers are also referred to as containers (in a more technical sense). So what are containers?

Java EE Container

Java EE containers are runtime environments for Java Enterprise Edition applications. As containers are implemented on top of JVM, they provide all services provided by JVM (to Java SE applications) and along with that they also provide services specific to the distributed environment. Java EE containers refer to two different types of containers known as Web/Servlet container and EJB container. They are classified in terms of what technologies they support. Let's go through both these containers in more detail:

Web Container:

Web containers run web applications which use Servlet, JSP and JSF pages and respond to HTTP calls from HTTP clients like browsers, mobile devices or other Java (SE or EE) applications. It can also host RESTful and SOAP web services. Now a subset of EJB 3.1 (added in Java EE 6), known as EJB lite can also be deployed in web containers (in fact EJB lite can also run on JVM). So, it intercepts HTTP(S) request from the client and then identifies, instantiates, initializes appropriate servlet/service endpoint; and then returns back the appropriate HTML page or response in JSON/XML/Text format. 

EJB Container:

EJB containers run enterprise applications made up of Enterprise Java Beans within the Application server. It abstracts business logic of your Java EE application and usually takes request from Servlets,  web services,  queue or other application servers. And just like servlet containers, EJB containers manages lifecycle of EJBs and provides services like transaction management, security, concurrency, naming services or ability to be invoked asynchronously.

Please note that Java EE compliant application servers support Servlet containers along with EJB containers; you can run full stack Java Enterprise Edition application. I have come across situations where people (loosely) refer to Applications servers as EJB containers. 
In the application server, web container and EJB container both share the same JVM instance. 

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