A deployment descriptor (DD) refers to a configuration file for an artifact that is deployed to some container/engine.
In the Java Enterprise Edition, a deployment descriptor describes how a component, module or application (such as a web application or enterprise application) should be deployed.
It also contains servlet/jsp wide settings as well as application wide settings for enterprise app of Java.
Another name of deployment decriptor is web.xml, resides in WEB-INF folder in the project hierarchy of Eclipse/NetBeans IDE.
Web.xml is a customized tag file. It allows to configure all the the settings in tag format. Such as, Initialization Parameter, Wel come file list for app, Cookies and Session tags etc.,