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For example, if you visit a site that has a YouTube video in one of its pages. This has been included by the website owner, using a piece of code provided by YouTube. YouTube will then be able to set cookies through this code, and know that you have watched that video, or even just visited the page the video is in.
Online advertising is the most common use of third party cookies. By adding their tags to a page, which may or may not display adverts, advertisers can track a user (or their device) across many of the websites they visit.
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Analytics cookies are probably the most common form of persistent cookies in use today.
However, persistent cookies can also , oddly, have a shorter life span than some session cookies, as they can be coded to be destroyed within a second or two of being set, whereas a session cookie will always last until you close down your browser.