XML Schema is an XML-based alternative to DTD.An XML schema describes the structure of an XML document.The XML Schema language is also referred to as XML Schema Definition (XSD).An XML schema is a description of a type of XML document, typically expressed in terms of constraints on the structure and content of documents of that type, above and beyond the basic syntactical constraints imposed by XML itself.

An xsd is used in an xml schema as a blueprint to define what can and cannot be included in an xml file.

the xml standard is maintained by a volunteer team at the w3c.

An XML schema is a certain type of XML document. It is expressed in constraints, on top of the normal XML constraints, on the structure and content of documents of that type.

An xml is normally validated against an xml schema, the schema is the rulebook that defines what can and connot be included in your xml file. You can also use xml validation software which will check and validate your xml, that sit is correct in terms of syntax etc and then check it agianst yuour schema.