What is the difference between xsd and xsi??

What exactly is the difference between XML Schema Document and XML Schema Instance ? xmlns:xsd="http://www.w3.org/...

xmlns, xmlns:xsi, xsi:schemaLocation, and targetNamespace??

For the following XML fragment: <project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://ww...