CSS is a style sheet language used to describe the presentation of HTML and XML documents, including how elements are displayed on screen, paper, or in other media. It allows control over color, font, size, spacing, and other properties that determine the appearance of elements. There are three main ways to apply CSS styles: internal style sheets, external style sheets, and inline styles. CSS rules consist of selectors that point to elements and declaration blocks that contain property-value pairs that describe the presentation of those elements. Comments can be added to CSS to explain code.