CSS is a language used to describe the look and formatting of a document. In most cases, HTML is the companion markup language for CSS. CSS is better at handling style because it's more flexible and feature rich than HTML.
CSS is a more efficient method of formatting documents because one style sheet can be applied across pages. For example, it speaks to HTML tags to render fonts, colors and weights according to whether the text is a headline or body text. This grants more flexibility and control in text formatting and reduces repetition, because it circumscribes the need to individually format every block of content in HTML.
Using CSS and HTML separately to tackle presentation and structure (respectively) can improve both accessibility and general usability. That's because better markup can improve the ability of a device or software to "see" the page, including text-to-voice software for the vision impaired, or text-only browsers for e-reading. CSS helps content conform to the specific operating system or device on which it's being viewed. The term "cascading style sheets" applies because multiple style sheets can be applied to a single page to prepare it for multiple scenarios. The style sheets are prioritized to avoid conflicts.