Ordinarily, websites can't read cookies other than the ones they've left themselves for security reasons.However, Ad-companies started displaying ads on sites depending on the content of the page. So, if you are a webmaster, you add a line of JavaScript code and a neat looking advertisement is displayed on your site page. Inorder to track you, as you moved from one site to another (both displaying sites ads from the same ad-company), the companies need to store cookies.When a user visits siteA.com, a cookie is not only set for siteA.com but also for a common domain which is accessed by all sites that use the ad-company’s code. Thus, siteA.com, siteB.com and all other sites displaying ads from adsite.com, store and access the same third-party cookie.