Google Analytics is Google’s free web analytics tool that mainly helps website owners to understand how their visitors engage with their website. It also sets cookies on the domain of the site you are visiting. It uses a set of cookies with names like ‘__utma’ and ‘__utmz’ to collect information anonymously and report website trends without identifying individual visitors.
Sometimes a cookie may be set on the domain of the site you are visiting. In the case of their DoubleClick product, a cookie called ‘__gads’ may be set on the domain of the site you are visiting.
Other Google properties, like YouTube, may also use the DoubleClick cookie to personalize ads.
Google also uses various conversion cookies to help advertisers determine how many times people who click on their ads end up purchasing their products. These cookies allow Google and the advertiser to tell that you clicked the ad and later visited the advertiser site. Conversion cookies are not used by Google for interest based ad targeting and persist for a limited time only. These cookies are generally set in the googleadservices.com domain, but may also be set in the google.com/ads domain. Conversion cookie data may also be used in combination with your Google account to link conversion events across different devices you use. Only anonymous conversion cookie data is shared with advertisers.
Some of their advertising products allow other companies to send their own cookies to your browser. For example, when you visit a page that uses DoubleClick, you may find that cookies are sent to your browser by Google and also by other parties. Each publisher and advertiser may work with various technology and service providers and each of these parties may set its own cookies, though each cookie can only be read by the domain that sets it. This means that several cookies may be sent to your browser by several companies in the time it takes a single web page to load. This is how most online advertising works.