CSS (Cascading Style Sheets) allows adding layout and design to websites in a new way and is essential for web design. CSS can be applied to HTML elements inline using the style attribute, or internally using the <style> tag placed in the <head> section. For example, to make the page background red with CSS, the code "body {background-color: #FF0000;}" can be placed within <style> tags or applied directly to the <body> element's style attribute.