Many companies know your personal browsing habits
Maybe today you read a post on your favorite blog and the article had a YouTube video available with it (which you didn't watch), and then you looked at a few friends' profiles on MySpace, and then you shopped for political books on Amazon. Of course, you know that the blog knows which article you read, and MySpace knows who your friends are, and Amazon knows the types of books you were looking at. However, even though you never used Google today, Google knows the article you read, the friends whose profiles you looked at, and the books you are thinking about buying.
How does Google know? Your browser told them.
When you read that blog post, the blog's website told your browser to request files from YouTube even though you didn't watch the video. YouTube is, of course, owned by Google. So, your browser dutifully let YouTube know which article you were reading as it grabbed files from YouTube (such as the still image to display to entice you to watch the video). And when you were on MySpace looking at friends' profiles, MySpace told your browser to request files from Google Analytics, a web traffic analysis service run by Google. So, your browser then told Google Analytics about each MySpace page that you visited. And then, when you were browsing for political books at Amazon, your browser was told to make requests to DoubleClick, an advertising company owned by Google, and Google got to know which books you are interested in.
Is Google evil? Are they the only one? Google wants to know everything that happens. They're amoral, not immoral. Gathering user data is useful to their company (targeted advertising is their primary source of revenue) and they'll use it if you give it to them. You just might not want to be part of their data set.
And Google isn't the only one, by far. Yahoo, Microsoft and many other smaller companies and websites have similar ways of knowing about people's browsing habits through their own ad networks and other services that rely on browsers making third-party website requests. "Social networking" sites gather information on your browsing habits without your consent when you visit a web page with embedded "sharing" buttons/widgets. Today's average web page is riddled with scripts, frames and other elements from third party sites, harming browsing speed, tracking you across the web, and often providing no added functionality.
Take back control. It makes sense that your browser would only be communicating with the website whose domain you see in the address bar at the top of your browser. However, as we can see, that's not the case. We can, though, with a little extra effort, take back control over which websites our browsers communicate with.
This browser extension, RequestPolicy, makes it so that you decide when your browser should obey commands by one website to make requests to other websites. By being able to decide this, you have more control over which companies know your browsing habits, your interests, and the great amount of other information that can be determined based on knowing how you spend your time online.