Third-party cookies are HTTP cookies that are not dropped by the sites you visit but by partner web services. Take, for instance, when you visit a site with Google AdSense enabled, you do not get only cookies from the site dropped into your browser; Google AdSense equally generates a cookie for you if you do not have one generated already. While the cookie by the site is first-party, that of Google AdSense is third-party. You could have an overwhelming number of third-party cookies you do not know of persistently saved on your hard drive.