As with standard cookies, third-party cookies are placed so that a site can remember something about you at a later time. Both are typically used to store surfing and personalization preferences and tracking information. Third-party cookies, however, are often set by advertising networks that a site may subscribe to in the hopes of driving up sales or page hits.
Third-party cookies are often blocked and deleted through browser settings and security settings such as same origin policy; by default, Firefox blocks all third-party cookies. Blocking third-party cookies does not create login issues on websites (which can be an issue after blocking first-party cookies) and may result in seeing fewer ads on the Internet.
Blocking third-party cookies increases user privacy and security but has created a problem for consumer tracking / ad serving firms, which often place ads that follow users around the Web. Combined with the removal of third-party cookies by other means, some firms estimate that 40 percent of all third-party cookies are removed.