Cookies are unique identifiers that web sites place on a user's computer, and are used to maintain persistent information regarding that user's activities on the site. Browser Cookies are kept in a file accessible to your browser, and may be accessed by websites in order to preserve login information and a history of activities.
Other types of cookie are not always as accessible. Flash cookies can be accessed and preferences edited through an Adobe site, which gains access to the files through your Flash player. They are not easily accessible otherwise. Many believe this to be a privacy issue.
In a series of documents obtained by Edward Snowden and published in December 2013, it was revealed that the American and British intelligence agencies NSA and GCHQ, used Google's PREF cookies as a way of pin-pointing target's communications for hacking purposes.
A PREF cookie is assigned to a user's browser every time they access a Google service (such as Google maps, Google translate or Google's search engine). The particular cookie allows information on preferences to be stored without including personal information of name or email address. It does however, allow the security agencies monitoring and tracking the information to identify the user's communications and thus enabling them to send software that can hack into the computer.