CSS (Cascading Style Sheet)
It is a style sheet language for specifying the appearance of text and other elements.
(X)HTML [(eXtensible) HyperText Markup Language]
HTML is a language for giving instructions for its layout on the page and marking a document containing cross-references that indicates its logical structure.
XHTML is a stricter and cleaner version of HTML.
XML (eXtensible Markup Language)
A specification that allows designers to create their own customized tags, enabling the definition, transmission, validation, and interpretation of data between applications and between organizations