I'm assuming you are talking about the ads you get when browsing. My son worked for a company that analysed every click made on the search engines. The ads you see are tailored to you and will change dynamically within seconds of you viewing a site/object. So if you are getting Harrods, you must be very posh.
Try it for yourself, search and view something that you would not normally look at (must be a major brand or product). See just how long it takes for related ads to appear.
1894 and big brother is here to stay.