We generate large amount of data while we are connected to the Internet. It not only records with whom we talk, chat or share content but also about the habits we have. The data about us being put into the corporate use.
For those who are uninitiated, its worthwhile to note that your login account is not the only thing by which you are being tracked. The advertisement serving websites also put up cookies on your computer to identify you. To record your clicking habits and serve you the advertisements that recognize what are the things that you are more inclined to buy.
One option to get away from this to install AdBlock in Firefox (and similar addon or plugin in other browsers) to turn off the ads alltogether but this hurts the websites that give you the free content.
Google with the option to opt-out of the profiling of the user habits and serve the independent ads only (link). However, this has not been so much publicised and very small percentage of people have done so. Additionally a plugin is also available to install with your browser to make the opt out permanent.
The best option is to visit National Advertising Initiative and opt-out of the target advertising from all kinds of ads serving services.