What is a cookie?
A cookie is a file that is stored on your computer when you visit a website. Most websites use them and they are generally harmless. When you revisit the website later or visit a different webpage a copy of the cookie file is sent to the website.
They can be used to store information and have many uses, with the most common including tracking, remembering your details or settings, and to keep you logged in to an account.
You can find out more information about cookies from the following websites:
When are cookies created?
Writing data to a cookie is usually done when a new webpage is loaded - for example after a 'submit' button is pressed the data handling page would be responsible for storing the values in a cookie. If the user has elected to disable cookies then the right operation will fail, and subsequent sites which rely on the cookie will either have to take a default action, or prompt the user to re-enter the information that would have been stored in the cookie.
Why are cookies used?
Cookies are a convenient way to carry information from one session on a website to another, or between sessions on related websites, without having to burden a server machine with massive amounts of data storage. Storing the data on the server without using cookies would also be problematic because it would be difficult to retrieve a particular user's information without requiring a login on each visit to the website.
If there is a large amount of information to store, then a cookie can simply be used as a means to identify a given user so that further related information can be looked up on a server-side database. For example the first time a user visits a site they may choose a username which is stored in the cookie, and then provide data such as password, name, address, preferred font size, page layout, etc. This information would all be stored on the database using the username as a key. Subsequently when the site is revisited the server will read the cookie to find the username, and then retrieve all the user's information from the database without it having to be re-entered.