In the ongoing EU investigation Google has offered to explain how they ranks their search results.
Our algorithms use hundreds of different signals to pick the top results for any given query. Signals are indicators of relevance, and they include items as simple as the words on a webpage or more complex calculations such as the authoritativeness of other sites linking to any given page.
The reason for the EU Commissions investigation is that three companies have complained to the EU claiming that Google has favored their own services on some searches. One of the companies behind the complaint is owned by Microsoft.