A deployment descriptor is a file that basically configures the artifacts that will be deployed, it is within the WEB-INF directory

Deployment descriptor in apache tomcat:

  application/WEB-INF/context.xml

Deployment descriptors in EAP

jboss-web.xml and web.xml

Deployment descriptor Glassfish

   sun-web.xml

Properties

Those files can be used to change properties on the application when deploying, for example, cookies configuration.