Google watches just about everything you do on the Internet with cookies that are planted in your browser.

As Google explains it:

“Many websites, such as news sites and blogs, partner with us to show ads to their visitors. To see ads that are more related to you and your interests, edit the categories below, which are based on sites you have recently visited.

Your interests are associated with an advertising cookie that’s stored in your browser. If you don’t want us to store your interests, you can opt out below.”

Want to see what Google has inferred about you, and edit the categories? Edit your Google ad preferences here.

 
 
Need to track what web pages have been visited - even if that was done in Safe Browsing mode or the browsing History was deleted?

Local Shared Objects (LSOs), otherwise known as “Flash cookies”, are so-named because they are a function of Adobe’s Flash. These cookies don’t even show up in your cookie or other privacy settings - they are buried deep within your Adobe Flash settings.  They are accessible only (or at least only easily) by visiting a special page at Adobe. 

http://www.macromedia.com/support/documentation/en/flashplayer/help/settings_manager07.html