An XML document posseses an hierarchical structure with two sections: Prolog and the entire body. The XML document starts by getting an optional Prolog, which includes XML declaration statement (<?xml version...), Processing Instructions, Document Type Declaration and Comments. We'll look at Processing Instructions (PIs) and Document Type Declarations later. Comments in XML document are exactly same as in HTML. Comments are used for some useful description/inline documentation. Comments begin with a <!-- and end with a -->.