To ensure private data protection, information about users is stored in cookies.
"HTTP cookie [...] is a small block of data created by a web server while a user is browsing a website and placed on the user's computer or other device by the user's web browser. [...] Cookies serve useful and sometimes essential functions on the web. They enable web servers to store stateful information (such as items added in the shopping cart in an online store) on the user's device or to track the user's browsing activity (including clicking particular buttons, logging in, or recording which pages were visited in the past)."
Source: HTTP cookie