valid document
1.It strictly obeys all prolog outline rules .
2. All the validity constraints identified in the XML specification

well-formed document
1. Some rules must be obeyed not all.
2. obeyed rules are not nearly as strict as those constraints required for validity.

DTD(Document type definition)acts as a rule book that allowes authors to create new document of the same type and with the same characteristics as a base document.

The IGNORE and INCLUDE keywords using by authors to turn portions of the DTD “on” or “off”.

XLink usesXml to define all the pieces necessary to build links into XML documents.

XSL (Extensible Stylesheet Language) is used to create extensible style sheets based on formatting rules.