Web pages have three key technologies on which they can draw. The big three are:
HTML (Hypertext Markup Language) - Provides structure to you document. Often described as providing semantic markup - the HTML should give structure and meaning to the content. For example a <h1> heading tag marks the content as a heading, both structurally and sematically. The HTML also describes the DOM (Document Object Model).
CSS (Cascading Stylesheets) - A complimentary technology to HTML, CSS's role is to style the document adding colour, setting fonts, laying out the design of the web page. CSS provides a set of selectors allowing the designer to identify different aspects of the document.