What is Deployment Descriptors? How many types of Deployment Descriptors are available? What are they?

According to war file
1. web.xml
2. ibm-web-bnd.xml
3. ibm-web-ext.xml

(or)
Deployment descriptor is an XML file that describes how to deploy a module or application by specifying configuration and container options. For example, an EJB deployment descriptor passes information to an EJB container about how to manage and control an enterprise bean

(Or)

This file is web.xml
This is an xml file. It tells to the server about the application.
Each web appication contain one deployment descriptor.