Official Google Blog: Introduction to Google Ranking
Google ranking is a collection of algorithms used to find the most relevant documents for a user query. We do this for hundreds of millions of queries a day, from a collection of billions and billions of pages. This same collection of algorithms is used for other Google search services, like Google Images, Google News, YouTube and Google Maps.
You might be thinking that this is something we all know, but what has divulged is that the Google ranking is broken down into three core parts:
1. Best locally relevant results served globally - The best relevant results are provided in what Google calls "no query left behind."
2. Keep it simple - Google likes to keep it simple without compromising on the quality of results.
3. No manual intervention - The rankings are powered by algorithms that reflect the behavior and linking patterns of the people in the whole web community.