External styles refer to a method of applying CSS (Cascading Style Sheets) to a HTML document by linking to a separate stylesheet file. This approach allows web developers to maintain a consistent look across multiple pages by defining styles in one location. When the external stylesheet is updated, all linked pages automatically reflect the changes, making it efficient for large websites. Using external styles promotes better organization and cleaner code, as the CSS rules are separated from the HTML content. This separation enhances readability and simplifies maintenance, allowing developers to focus on design without cluttering the markup with style information.