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These cookies are used to deliver adverts more relevant to you and your interests. They are also used to limit the number of times you see an advertisement as well as help measure the effectiveness of the advertising campaign. They are usually placed by advertising networks with the website operator's permission. They remember that you have visited a website and this information is shared with other organisations such as advertisers. Quite often targeting or advertising cookies will be linked to site functionality provided by the other organisation.
More about cookie classifications
First Party Cookies
One of the key attributes of a cookie is its 'Host' - this is the domain name of the site that ultimately sets the cookie. Only the host domain can retrieve and read the contents of the cookie once it has been set.
If the host name is the same as the domain in the browser address bar when it is set or retrieved, then it is a First Party Cookie.
First party cookies are only set or retrieved by the website while you are visiting it, so they cannot normally be used to track activity or pass data from one site to another.
However the owner of that website can still collect data through their cookies and use that to change how the website appears to the user, or the information it displays.
Most desktop browsers allow you to see a list of the cookies that have been set – and they will normally be listed by the host domain value.
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.
Persistent cookies are created by giving them an expiry date. If that expiry date is reached, it will be destroyed by the computer. If the expiry date is not set then it is automatically a session cookie.
The expiry date will normally be saved as the time the cookie was first created plus a number of seconds, determined by the programmer who wrote the code for the cookie. However, there is no real limit on the expiry date - so it could be set to be 20 years in the future. In addition, if you revisit the website that served up the cookie, it may automatically place an updated version on your computer - with a revised future expiry date.
If you login into a website, then shut down your computer, start it up again, and go back to the website to find you are still logged in - then it is using a persistent cookie to remember you.
Persistent cookies are also used to track visitor behaviour as you move around a site, and this data is used to try and understand what people do and don't like about a site so it can be improved. This practice is known as Web Analytics. Since Google started providing its own analytics technology free of charge to website owners, almost all websites use some form of it - although there are also paid-for services available to rival Google's.
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.