A stylesheet is a collection of rules that dictate how a document, typically written in HTML, should be presented on a web page. The most common type of stylesheet is a Cascading Style Sheet or CSS, which allows developers to control the layout, colors, fonts, and overall appearance of a website. By separating content from design, stylesheets make it easier to maintain and update web pages. Stylesheets can be applied in three ways: inline, internal, and external. Inline styles are added directly to HTML elements, internal styles are included within the document's head section, and external styles are linked from separate CSS files. Using external stylesheets is often preferred for larger projects, as it promotes consistency and reusability across multiple pages.