CSS (Cascading Style Sheets) is a styling language used to define the look and feel of HTML documents. It is used to add color, font, background, borders, margins, padding, and other visual styling to webpages. CSS allows web designers to create complex layouts, such as fluid layouts and responsive design. It also provides the ability to control the presentation of multiple webpages within a single website. CSS is written in documents (called stylesheets) that are separate from the HTML documents they are used to style. This keeps webpages organized and makes them easier to maintain.
HTML is not the same as CSS. Not even similar.