HTML (hyper text markup language) is used mainly for building websites and other web content. You can add text and pictures to your webpage using this.
CSS (cascading style sheets) is used to design and make the website easier to read.
A webpage without CSS would be quite boring, since CSS allows you to customize colors, position elements, and bring your webpage to life. A non-CSS styled page would just be a white page with text on the side (and maybe some links, too?)