Creating a comprehensive style guide when designing for the web is a great way to define the look and feel of a site/app. As a minimum your style guide should cover global typography styles, colour palettes, grid layouts, and other core design elements that will make up the pages on your site.
Gone are the days of creating a website design page by page.
Ultimately a well thought out style guide will save you time and will act as a handy reference point for developers and designers alike. It will require continual iteration but should inform the design of new features and will ensure that as your site grows it maintains a visual flow and design consistency, in keeping with your brand.
Take a look at BBC's GEL guidelines http://www.bbc.co.uk/gel,
a fantastic example of a carefully crafted style guide, created for one of the biggest and best known brands in the country.