There are four types of J2EE component:
Enterprise Applications:-
              These components provide a top-level structure which contains any number of other J2EE
components (except other J2EE Enterprise Applications). The JAR file holding an enterprise application has the suffix .ear, so Enterprise Applications are frequently referred to as EARs.
Web Applications:-
              These J2EE components may contain Servlets,Java ServerPages, Tag Library Definitions and plain Java classes or resources as required for proper operation. The documents and Java classes of Web Applications frequently create markup language (HTML, WML, XML, etc.) responses to requests from web browsers or business applications. The JAR file holding a Web application has the suffix .war, so
WebApps are frequently referred to as WARs.
Enterprise JavaBean Applications:-
              These components hold server-side business logic packaged within Enterprise JavaBeans.
Resource Applications:-
              These J2EE components hold Java classes that act as drivers or communication gateways to Enterprise Information Systems (EISs). It is highly likely that the manufacturer of an EIS packages its driver into a J2EE resource archive (RAR).

Some firewalls permit only email traffic through them, thereby protecting the network against any attacks other than attacks against the email service. Other firewalls provide less strict protections, and block services that are known to be problems.

Generally, firewalls are configured to protect against unauthenticated interactive logins from the ``outside'' world. This, more than anything, helps prevent vandals from logging into machines on your network. More elaborate firewalls block traffic from the outside to the inside, but permit users on the inside to communicate freely with the outside. The firewall can protect you against any type of network-borne attack if you unplug it.