What is a cookie

A cookie is a text file, that's all, website designers will use cookies for a variety of reasons.
 
If you go to a website that allows you to customise a page so that it looks the same every time you visit it, the designer will store these pieces of design information in a Cookie.
If you become a member of a site that wants to make your navigation easy, they will store your membership info in a Cookie so that you are instantly recognized when you visit the site.
Controlled access can be achieved via a Cookie so that you can't view pages unless you have signed in with a UserID and Password, usually these cookies expire after you leave the site.
There are many ways a cookie can be used, from simply seeing what pages you visit to allowing you to use an on-line shopping cart!

It will also give the designers of the site valuable information regarding the pages you vist, he can then use that information to improve the pages most visited.