What is deployment descriptor

Deployment Descriptor is a XML document with .xml extenion. It basically descripes the deployment settings of an application or module or the component. At runtime J2EE server reads the deployment descriptor and understands it and then acts upon the component or module based the information mentioned in descriptor.  
 
For example EJB module has a deployment descriptor ejb-jar.xml where we mention whether it is session or entity or mesage driven bean and where is the home, remore and Bean classes are located and what type of transaction etc. In a simple word, without deployment descritor the Container ( EJB/Servlet/JSP container) will not know what to do with that module.

Deployment Descriptor is a XML document with .xml extenion. It basically describes the deployment settings of an application . At runtime webcontainer reads the deployment descriptor and understands  which .class file is mapped to the url-patteren name based on the information mentioned in descriptor.  the webcontainer calls that file