Cookies are small text files that are placed by a website on a visitor’s computer, which is then readable by that website at a later point in time.
They are not programs, they are not by themselves malicious and they can be very helpful. They are mainly used to help recall something about the user, their visit, their preferences and are very common to most websites. For example they are used to remember usernames/passwords on many websites, preferences for country location or language, and they are critical for wishlists and shopping baskets when using an e-commerce site.
They are often used for analytics, i.e. to monitor visitor behaviour while on a website. This means that website owners can check which pages are the most popular, which get visited first, how a visitor navigates a site, how long they visit and more. This is generally used to improve site design but can be used for marketing by the website owner.
Cookies can also be used by advertising and marketing companies to track your visits, what adverts you see and whether you click any adverts, but also to offer targeted marketing, that is adverts which are more relevant to you.
The most common types of cookies are:
Session cookies – Often used for 'Shopping Cart' and to remember which pages you visited, or for when you use the 'Back' button. Once you close your browser or log out, the cookie will expire and be deleted.
Persistent cookies – Can be used to store information or settings about you to improve your experience on a website. For example, “Remember me” cookies which store your username and password when you visit a site. This is not used to identify you: only to make logging into a website faster and more convenient. They can also be used for wishlists on e-Commerce websites like Amazon.
Third Party cookies – These are cookies placed by another website or service i.e. it has a different address to the one you see in your browser’s address bar: in other words a third-party. Examples include cookies created by Google's services (Google Analytics, Google AdSense) and those created by advertisements served from a different website (which is very common). NB. All modern browsers provide a setting that allows a user to block third-party cookies.