A stylesheet is a set of CSS rules used to control the layout and design of a webpage or document. Internal stylesheets are placed inside a
<style> element inside the
<head> of a web document, and external stylesheets are placed inside a separate
.css file, which is applied to a document by referencing the file inside a
<link> element in the document's head.
External stylesheets are generally preferred because they allow you to control the styling of multiple pages from a single place, rather than having to repeat the CSS across each page.
- CSS first steps
- Stylesheets on Wikipedia