Third-party Cookie Crumbs
“A small amount of text stored in the user’s computer that is created by a Web site with a domain name other than the one the user is currently visiting. By default, third-party cookies are often allowed by a Web browser; however, they may be blocked, as they are widely used by advertisers to track browsing history.”
This is TheFreeDictionary’s definition of a third-party Cookie. Interestingly, there is a mentioning that these are commonly allowed by default web browser settings and can be blocked. Now why is this so relevant that it fits into the definition itself one might ask? Let us see if that becomes more obvious after looking at what a third-party cookie actually is. Assuming that you already know what first party cookie, or just “cookie”, is the difference is that the third-party cookie is not coming from or exchanging information with the website the user is actually visiting. Thus the user does not actively pursue the cookies exchanged with third-party servers and may therefore be totally unaware of that there even is a connection to the third-party server in question. See below how these cookies work, credits to Mdscott and IT Law Wiki for the illustration.
Back to why there is a remark about allowing and blocking third-party cookies in web browsers. From the user’s point of view, the common and most apparent effect of third-party cookies is that the advertisement is tailored to the user (actually web browser). In effect, the third-party server gathers information about the user’s online habits. What sites are visited, what pages on that side is accessed, what links are clicked on, which ads are clicked on etcetera. By having this data, the third-party server can make better guesses on what ads might suit the user. Many people find it unpleasant to realize that the ads on sites are customized to their unique profile and that an unknown party somewhere knows so much about what they do online. Lately, it seems that it becomes more common to turn off third-party cookies. (However, there are surely a lot of web surfers out there who do not know what third-party cookies, or any kind of web browser cookies really.) In the context of this “enlightenment” trend, the note in the definition on blocking and allowing cookies from third party domains proves to be relevant.