XML (or eXtensible Markup Language) is a way of writing documents that are both human and computer readable.
The syntax is a lot like the HTML format, as both are based on the SGML standard. XML documents require a closing tag for each opening tag though, and don't have a predefined set of tags, instead it leaves it up to the user to define the tag names.
The advantage of XML is that it's human readable and writable, and it doesn't suffer from byte order issues between different platforms. The downside is that it a lot more verbose than binary formats having quite some overhead over them. Besides that, text based formats are not as fast to process.
An XML schema is an xml document that defines the format and tags of another XML document. XML Schemas are available for most XML Based standards.