The question: What’s a third party cookie? OK, let’s assume I’m an expert at these things, which I’m not, but let’s just assume that I play an expert at these things. Here’s your answer….
Cookies are small pieces of information stored in your browser’s cached files. If you visit a site, say example.com, that site might decide to store some state on your browser — a cookie. The cookie is a 1st party cookie because it is created and sent back and forth between your computer and that site, example.com, in your browser’s url field.
Let’s look at this a little more closely. Imagine you browse to http://example.com/hello.html. This site might send back a 1st party cookie: FOO=1. The cookie could be stored in the domain of example.com, and only visits to that example.com would prompt the browser to automatically send that cookie value to example.com on subsequent visits. Because of the rules around cookie security, however, your browser would never send an example.com cookie to another site like example2.com.
Now imagine that the example.com/hello.html file has additional links to other sites. Maybe hello.html has an image link that pulls a photo from example2.com. Now example2.com is being called from your example.com/hello.html document. Your browser will not send any example.com cookie values to the referenced example2.com site. However, the example2 site may decide to drop a cookie as well. Since it is not the primary site of your document, which is example.com/hello.html, the example2.com cookie is called a 3rd party cookie.
Hmmm… so here we have examples of 1st party and 3rd party cookies. So, what’s a 2nd party cookie? I don’t actually know the answer to that question. If the site of your document is the 1st party and referenced sites are 3rd party, that really leaves the browser or its user as the 2nd party. Maybe a 2nd party cookie (which I’ve never really heard in any discussion) would simply be a cookie manually created by the user? Hmmmm…. probably not an important point. However, hopefully you know the difference between 1st and 3rd party cookies!