What is the difference between xsd and xsi?

http://www.w3.org/2001/XMLSchema

The Simple Version : This is the namespace used within an XML Schema (XSD). An XML schema is used to describe what's valid within an XML instance document.

The Less Simple Version : This is the namespace of an XML Schema that describes the structure of an XML Schema. In other words a schema that describes itself.

An XML Schema (XSD) must be written using the types defined within this schema.

For Example.

<xs:schema elementFormDefault="qualified" xmlns:xs="http://www.w3.org/2001/XMLSchema">
    <xs:element name="MyElement" type="xs:string" />
</xs:schema>

http://www.w3.org/2001/XMLSchema-instance

This is a namespace used within XML Instance documents to provide additional data to the XML Parser that is processing it. It describes the attributes xsi:schemalocation, xsi:noSchemalocation, xsi:type and xsi:nil which the XML parser can use to assist it with validation.

For Example.

<MyElement xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
           xsi:noNamespaceSchemaLocation="MySchema.xsd">
    string
</MyElement>

xsd and xsi Similarities

• Both are XML namespace prefixes, abbreviations for an XML namespace.
• Both are, as are all namespace prefixes, arbitrarily named; other namespace prefix abbreviations could equally well be used. However, both prefixes are conventional and therefore recommended. (An also-conventional alternative to xsd is xs.)

xsd and xsi Differences

• The xsd (or xs) prefix referring to the Schema Namespace (http://www.w3.org/2001/XMLSchema) is used in XML Schemas (XSDs) for the elements, attributes, and types of the W3C XML Schema Recommendation itself. (This is possible because XML Schema is itself XML.)

• The xsi prefix referring to the The Schema Instance Namespace http://www.w3.org/2001/XMLSchema-instance is used in XML document instances for several special attributes defined by the XML Schema Recommendation:

  • xsi:type allows an XML instance to associate element type information directly rather than through an XSD. See How to restrict the value of an XML element using xsi:type in XSD?

  • xsi:nil allows an empty element to be considered to be valid when the XSD might not otherwise have allowed it.

  • xsi:schemaLocation and xsi:noNamespaceSchemaLocation provide hints to the XML processor as to how to associate an XSD with an XML document. Use xsi:schemaLocation when there is a namespace; use xsi:noNamespaceSchemaLocation when there is no namespace.

See Also

• Namespace related attributes in XML and XML Schema (XSD)
• How to restrict the value of an XML element using xsi:type in XSD?