Are you connected to the internet by Virgin Media, BT or Talk Talk? The you had better watch out.
Apparently all three have contracted to extract the details of every web page you look at and sell this to an 'advertising broker'. So you can get 'targeted advertising'. If you don't like this idea, Talk talk will let you opt out.
The company doing this is called Phorm.
There is a lot more on this, and a chance to take action here.
You really should read all about this.