CSS (Cascading Style Sheets) allows control over the visual design and layout of web pages, including colors, fonts, spacing, sizes, backgrounds, and formatting for different devices. CSS saves time and improves page load speeds compared to HTML alone. There are three main ways to apply CSS - inline within HTML tags, internal using <style> tags, and external linking a .css file. CSS properties specify various visual attributes like colors, borders, backgrounds, and font properties to control the presentation of HTML elements.