At their core, cookies are simple plain text passed from a website to your computer and stored by your browser for later use. That text is passed back to the server when you request a web page, and it’s used by developers for an array of tasks. For example, when you click “remember me” as you log in to a site, a cookie is set so the site doesn’t ask you to log in again. Without cookies, the internet would be much more annoying and forgetful. But that same useful technology also allows other sites, services, and advertisers to invisibly track you.
He explains the difference between a first-party verses a third-party cookie.
Third-party cookies, are placed by advertisers and marketing companies to track you across websites, allowing them to figure out who you are even as you leave the original site that set that cookie.
Although there is a significant push to limit third-party cookies, there is still no consensus on what a solution looks like.
In another post, Ariel Bogle explains why the removal of third-party cookies only strengthens Google’s position in the ad market as they are able to collect data associated with users of their own applications.