A document type definition (DTD) defines a single set of element and attribute markup that can be thought of as a single tag language, vocabulary, or namespace. XML namespaces let multiple such languages contribute to a single document, even though DTDs do not have a mechanism to allow this. XML namespaces associate element and attribute names used in XML documents with specific URIs (Uniform Resource Identifiers). Doing so allows you to, for example, distinguish between <title> elements that one namespace uses to indicate document titles and another namespace to indicate formal titles such as Doctor and Professor.

You can find a full description of namespaces, their syntax, and their intended use at the World Wide Web Consortium (W3C) Web site (http://www.w3.org/TR/Rec-xml-names).