When it comes to Retrieval System (Information Retrieval), which search engines rely on, two critical components should be mentioned to make up the major of search engine functionality: Relevance and Popularity.
1. Relevance is usually defined by Document Analysis, which is an algorithm to determine the quality of documents on your site.
2. Popularity is defined by Links Analysis, which is an algorithm to determinehow other web sites/pages think the quality of documents on your site. These two algorithm then determine the score of your document, the ranking of your web pages.