A style sheet is a template for a page where fonts and colors, etc ... are predefined for required sections. These style sheets can then be used in other web pages. Use style sheets to establish a unique look. Then use a style sheet as a template for all your web pages and thus when changing a single thing in a style sheet one changes all pages using the sytle sheets you have previously defined.

This is heading 1

This is a paragraph

1. Item 1
2. Item 2
3. Item 3

This is heading 2