Cascading Style Sheets (CSS) allows obtaining full control over HTML elements and their default properties. CSS can be used to easily redefine properties of any HTML tag, opening new design opportunities. Styles defined in CSS can be reused throughout an HTML document or across multiple pages for consistent formatting. The document discusses different methods of implementing CSS, including inline, internal, and external stylesheets. It also covers various CSS properties for formatting text, fonts, colors, backgrounds, lists, borders, opacity, and more. Examples are provided to demonstrate different CSS declarations.