CSS (Cascading Style Sheets) allows you to define styles for HTML elements. There are three ways to insert CSS - external style sheets, internal style sheets, and inline styles. CSS provides benefits like saving time, design flexibility, faster page loads, and easy maintenance. CSS syntax and tags are used to style text, fonts, links, lists, tables, boxes, positioning, and more. Examples demonstrate how to style paragraphs, inputs, tables, grouping elements, and nesting selectors.