CSS (Cascading Style Sheets) is a style sheet language used to describe the presentation of HTML documents, including how elements are laid out on the page, their colors and fonts. CSS saves a lot of work by allowing control of layout and styles across multiple web pages from one stylesheet. CSS declarations are made up of selectors that point to HTML elements, properties to specify element display properties, and values. There are three types of CSS: inline, internal/embedded, and external.