A Style is a group of formatting attributes that controls the appearance of elements on a page that uses a template with the style applied. The advantage of using a Style is that in addition to being linked to multiple pages, when a Style is updated or changed, the formatting of all the pages that use that Style are automatically updated as well.
Creating Styles requires knowledge of Cascading Style Sheets (CSS) and how different attributes affect the look and feel of your site. A good starting point for understanding CSS is located at http://www.htmlhelp.com/reference/css/.
Styles are published separately from templates and pages. This allows you to manage different aspects of your content independently.