Ok, so I'm not quite sure the difference between these languages. Could someone clarify? I know that XML has user-defined tag and html is pre-defined, but thats basically the extent of my knowledge.

I know that HTML5 is supposed to replace HTML, but wasn't XML supposed to do that as well? Basically, which languages here are a substitute for the other, and which complement? Does XML replace XHTML?

HTML is the HyperText Markup Language, which is designed to create structured documents and provide for semantic meaning behind the documents. HTML5 is the next version of the HTML specification.

XML is the Extensible Markup Language, which provides rules for creating, structuring, and encoding documents. You often see XML being used to store data and to allow for communication between applications. It's programming language-agnostic - all of the major programming languages provide mechanisms for reading and writing XML documents, either as part of the core or in external libraries.

XHTML is an XML-based HTML. It serves the same function as HTML, but with the same rules as XML documents. These rules deal with the structure of the markup.