HTML and CSS are used to structure and style web pages. HTML provides the content layer through elements like headings, paragraphs, and lists, while CSS controls the presentation with properties that define styles for different elements and tags. CSS selectors allow styles to be applied to HTML elements based on their element name, class, ID, and relationship between elements. The box model is also an important CSS concept, describing the content, padding, border, and margin areas of blocks and inline elements.