What is a deployment descriptor in WAS?

A deployment descriptor (DD) refers to a configuration file for an artifact that is deployed to some container/engine.
In the Java Platform, Enterprise Edition, a deployment descriptor describes how a web application or enterprise application should be deployed. It directs a deployment tool to deploy a module or application with specific container options, security settings and describes specific configuration requirements. XML is used for the syntax of these deployment descriptor files.
For web applications, the deployment descriptor must be called web.xml and must reside in a WEB-INF subdirectory at the web application root.
For Java EE applications, the deployment descriptor must be named application.xml and must be placed directly in the META-INF directory at the top level of the application .ear file.