What is the difference between xsd and xsi?

`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.)

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.