The first generation based the search on what was on the Web page. Important factors, like keyword density, title, keywords density. Meta tags had an important role, keywords in the domain name, and also some keywords in the URL. Search engines started looking like yellow pages.
The second generation based the page ranking on related links; but it looks like the days of huge link exchange programs are over.
The third generation is already underway, adding word stemming and a thesaurus on top of the term vector database to assist in keeping a search in context. The 3rd generation search engines will build personal profiles, based on past searching habits and the page vector (the keyword density per page).
