Michaela Campbell, author of popular books, defines a theme engine as “what you say about your Web page, how the structure of other people’s Web pages compares on the same topic, and what other people say your site is about, to be in harmony with each other, be as one.

A theme engine looks at all the information on a seed set, or a group of sites and pages that it has already spidered and has in its index. It assigns each page in the index a number or page vector. This becomes the core of the search engine.

Then the search engine adds and calculates words and incoming links to the page, making sure they match up to the term vector, what the search engine has determined that the page is about, must match what the rest of the Internet says your page is about in their links to you. The next step is to establish the stats and cache data. If the site is one of a search engine’s top exit pages, it must be good, because people don’t come back and search some more once they have found your site.  If the site gets searched and clicked on so often that it is in the engine’s cache for speedy data retrieval, the site must be very good indeed.

The idea is the theme based search engine is looking for unanimous approval that the site is about a particular topic; the more narrow the focus on that topic, the better the site will do. In Campbell’s opinion, all search engines are moving toward being theme based.