How Google use recommendations?

In this post, I would like to talk about the recommender systems, and give a brief introduction of how Google use recommendations. The definition of recommender systems is to form or work from a specific type of information filtering system technique that attempts to recommend information items(movies, TV program/show/episode, video on demand, music, books, news, images, web pages, scientific literature such as research papers etc.) that are likely to be of interest to the user.

Google is the one of most successful Internet companies in this area, and uses recommendation technologies to improve its search product. There are several ways that Google does this:

1.Google customizes your search results "when possible" based on your location and/or recent search activity;
2.When you're signed in to your Google Account, you "may see even more relevant, useful results based on your web history";
3.Google's search algorithm PageRank basically depends on social recommendations - i.e. who links to a webpage;
4.Google also does item recommendations with its "Did you mean" feature.

Reference:
http://en.wikipedia.org/wiki/Recommender_system
http://en.wikipedia.org/wiki/Google