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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.
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As the name suggests, this type of cookie is saved on your computer so that when you close it down and start it up again, it can still be there.
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Analytics cookies are probably the most common form of persistent cookies in use today.
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