An XML schema is a document that defines how an XML document should be structured. An XML document is said to validate against a schema if it correctly follows the schema's rules and definitions. Schemas are themselves written in XML and use the file extension ".xsd". For example, there are a number of schemas that define different flavors of XHTML.
This page allows you to validate an XML document against a schema. It will tell you if the XML file passed or failed validation. It uses the DOMDocument PHP class to do the validation.