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.