The new policy means that Google can pool information acquired from monitoring users' online activity across all of its products. This information is then used to direct advertisements to its users.
The changes mean that Google can use information gained in one application - such as Gmail - and use it to shape other services. While this brings new levels of personalisation, it also allows advertisers to target their messages more accurately.
Investigators in France doubt the legality of the policy and have launched a European-wide investigation.
French privacy watchdog CNIL wrote to Google days before the policy was introduced, saying that, along with EU data authorities, it was "deeply concerned about the combination of personal data across services".
CNIL added that it and the EU data authorities "have strong doubts about the lawfulness and fairness of such processing, and its compliance with European data protection legislation".
Many blogs and forums have issued advice to users who want to learn more about Google's changes. A popular suggestion has been to log into google.com/history, where users can delete their browsing and search history.