The World Wide Web forces us to recognize this as a big problem. The W3C is a leader in this effort. A resource description community is characterized by common semantic, structural and syntactic conventions for exchange of resource description information. A markup language is a mechanism to define tags and the structural relationship between them in documents. Since there is no such thing as an XML DTD the SGML DTD must be used for this purpose, at least until the XML schema now in progress is completed.
Schemas enable resource communities to define and exchange vocabularies.
SGML is character-based. For XML we need to show different types of data types, e.g., ISO date, floats, document numbers. This enables it to handle many different kinds of characters but also makes it case sensitive. Often syntax is not enough and a common structural representation for expressing statements is required. The basic structural element of RDF is the RDF statement and there is only one way to express a statement.