Cookies
When browsing a website, small pieces of information is sent between the website and the user - a cookie. This information is stored as a text file on the users computer, and is sent to the web site when performing a page view. The web site can send updated cookies to the web browser when the page is being displayed.
There is no generic description of what a cookie contain; but it is often identifications, history, settings, or a shopping cart. This is what makes the web site able to identify their visitors, for example as a logged in user or as someone that has clicked a "Like" button. There are two different kinds of cookies, first party cookies and third party cookies. The first party cookies belong to the web site visited, and third party cookies belong to the web sites' partners.