EAR (Enterprise ARchive) files can contain web components (WAR), EJBs (EJB-JAR), any other libraries (JAR) and application.xml (of course). It's a standard way of packaging enterprise applications for deployment.
To answer the poster's question, I'd say that you don't need to know about EAR. Section 2 of the exam objectives is :
Section 2: The Structure and Deployment of Web Applications
* Construct the file and directory structure of a Web Application that may contain (a) static content, (b) JSP pages, (c) servlet classes, (d) the deployment descriptor, (e) tag libraries, (d) JAR files, and (e) Java class files; and describe how to protect resource files from HTTP access.
* Describe the purpose and semantics of the deployment descriptor.
* Construct the correct structure of the deployment descriptor.
* Explain the purpose of a WAR file and describe the contents of a WAR file, how one may be constructed.
I don't think you are supposed to know about EAR. (although it won't hurt to know about it ;) )