As we are all well aware, search engines consistently update their ranking algorithms. In fact Google updated there is 528 times in 2012! One aspect that is consistent and probably most important is your site's theme or relevancy to subject matter.
When the search engines scan your Web content they identify subjects being to your site. In simple terms they look at how strongly focused your site content is on certain subject matter(s).
One of the means by which you can improve the ranking of your site is to have a site that is very clearly and strongly focused on a single subject matter.
How Do Search Engines Determine Theme
By examining the content and how strongly that content is about particular subjects, the search engines will determine the overall theme of your site. This is largely driven by the relevancy of that content to a particular subject matter.
All content you have around a particular subject and the use of words relating to that subject, the higher the search engines gauge your site relevancy to that theme.
Website Structure Can Influence
A well-structured website that strongly links content: two subject can also assist the search engines in determining the theme(s) in your site.
Linking related content pages from each other strengthens the bond or level of cohesiveness. When putting pages into your site be sure that links within page exists to other pages within your site where the content of those pages is closely aligned to the current page. Don't simply rely on your menus. Generally these require you to traverse the path down and up following links to take you to the required page. Links within your page provide both visitors and search engines easier ways to find related content. This therefore is not only assisting with your search engine ranking but also providing a better user interface or visitors to your website.