Web cookies are stored primarily on the client side, but the server operator has the option of obtaining their own copy of these so-called first-party-cookies when the cookies are sent back by the browser. First and foremost, cookies ensure a more user-friendly web experience, which records the user and their surfing habits and adapts the visited websites and web functions to them without being asked.
However, the data stored in cookies is also very interesting for website operators, as they can derive statistics and draw conclusions about the surfing behavior of visitors. Furthermore, server-side cookies enable the creation of user profiles. It is these user profiles that enable targeting – which is the basis for personalized advertising.
In this context, so-called third-party-cookies are particularly effective: they are usually set unnoticed by third parties and spy on the surfing behavior of users, usually over a long time and on different servers. For example, if you visit health-related websites frequently, you are likely to see more advertisements for drugstore items– even on websites that have nothing to do with the topic. Another user is likely to see different advertising on the same website because their user profile reveals interest in a different subject area. Data protectionists therefore see cookies as the main cause for “transparent users” whose traces on the internet are misused for marketing purposes such as behavioral targeting.
Some users probably hadn't even noticed the existence of cookies until the new EU Directive 2009/136/EC came into action, whereby website operators require their visitors to be informed about the storage of user-relevant data. Since then, when you visit European websites for the first time, you must accept the use of cookies. With one click or further use of the website you then agree that your data will be stored – both locally on your computer and on the server side.