there are 3 key basics in the site's composition

1. URL
2. Link Back to Parent Page / Category / Sub-group with suitable anchor text.
3. With Proper composition in position, you can build back links with related keywords,

SEO is a means to tell Google what your website content is about, with right anchor links and excellent on page structure, you can simply target a lot of related keywords without truly having them on your site. Google had a very controlling content analysis relevancy engine, it will show your content for many long tail keywords that you never expected, and there for increase your web traffic obviously!