Google was caught last week circumventing the built-in privacy settings in Apple's Safari browser to install third-party cookies on users machines. Third-party cookies are typically used to track user web surfing habits across all web sites, not just the site that installed the cookie. The exploit could have allowed Google to follow all Safari users' web activity.
Safari is the default browser on all Macs and iOS devices. There's also a PC version available.
Google claims the workaround was designed to allow its "+1" buttons to serve up personalized advertising for users with accounts on the search engine. Obviously, given the way Google achieved the third-party cookie installation and the lack of disclosure is concerning to privacy advocates.