Cookies are text files stored on your computer that web sites use to keep track of information their site requires. This can be as simple as a placeholder that indicates for you what you have already seen on that page (usually by changing the text color) or remembers your preferences. These cookies have no contact with anyone since the info they contain is meant solely for your benefit.
However, some companies use those cookies to track where you have been and what you have done. The difference depends on whether the cookie is first party or a third party cookie. Third party cookies are set not by the web site you are viewing, but rather by a site located elsewhere. This is the case with most advertising banners. Of course, there are also companies that outright abuse the technology in order to track web surfers all over the internet.
One such company is advertising giant DoubleClick. Cookies, by design, are meant to be accessible only by the site that sets them. This is to keep one web site from reading the cookies set while a person is on another site. DoubleClick exploits a loophole by running ad banners from its own servers, and using those servers to set and read cookies.
DoubleClick has ads on thousands of web sites and can read any cookie set by any of them. In this manner, DoubleClick uses these cookies to track web surfers from one web site to the next the same way a rancher brands his cattle and tracks their movement across on the plains.