When I create a XML document,I say that I create an XML instance from a XML markup language.But a markup language is defined by a set of rules which should be followed by the XML documents created correspondingly.And XML markup languages are defined by the schema.So,what about the XML documents created without a schema?What can we say about the markup language of those XML documents who do not have an associated markup language?
There are XML-based documents that conform to an explicit language (aka vocabulary) which is specified in an XML Document Type Definition (DTD) or an XML Schema. Here the instance document must conform to the rules specified in the custom language AND the rules specified in the XML specification for well-formedness.
There are also XML-based documents that conform to the rules specified in the XML specification for well-formedness only. In this case, there isn't an "explicit" definition of a custom language, but one still exists. In this case there is more flexibility in regards to how the elements and attributes are organized as there is no "defined" structure besides the rules of well-formedness which are specified in the XML specification.
Keep in mind that every XML-based language does not have a name. Some examples of XML-based languages with names are: Business Process Modeling Language (BPML), Financial products Markup Language (FpML), Simple Object Access Protocol (SOAP), Web Services Description Language (WSDL).
Examples of XML-based languages without a name are the Java Servlet configuration data which goes in a web.xml, or Struts configuration data which goes in struts-config.xml.
I think you (Sudhanshu) have it all backwards. The vast majority of XML documents aren't described by a schema. It's perfectly normal for people to agree on the meaning of a document without writing a schema. And really a schema doesn't describe the meaning of a document either, it just specifies the names of elements and attributes and where they can appear. It describes the format but not the content.
So in the end, the meaning of each XML document is specified by an agreement among the people who use it. Schemas are far from being a complete implementation of that process.