This document introduces Cascading Style Sheets (CSS) and discusses its syntax, selectors, and different types including inline CSS, internal CSS, and external CSS. CSS is used to style web pages and control layout, and has benefits like easier maintenance and faster page loads. CSS syntax uses selectors to point to HTML elements and properties to define styles. The three types are inline CSS using the style attribute, internal CSS within <style> tags in the head, and external CSS linking to a separate .css file.