A Cookie, a small, usually harmless, piece of data is placed onto you harddrive into a file. Cookies come in two types, so-called session cookies which persist on your drive untill the browsing session expires or until you close all your browser windows. These cookies are essential to some sites like online shops. You shop, place items into your basket, place more items into your basket, maybe remove some and finalize your sale. This is all made possible by the session cookie, if it wasn't there you'd end up having no items in the basket when surfing to another product category. This happens, because the site 'sees' you as a new visitor without the session cookie, the session cookie tells the site you're not a new visitor and 'forgets' your executed actions. Thus, the session cookie identifies and remembers you while having your browser running to provide extra functionality. The session cookie can also be used to generate user satistics, what pages did the user visit for example. To finalize the session cookie: these pose little threat and most of them are needed to let sites function properly.


The persistent cookie will not be deleted when closing your browser, but it will expire, all cookies have an expiration date though some cookies can be present for months. This type of cookie can also perform a usefull function. If you don't want to enter your username, password or both, when accessing a certain site, a persistent cookie can store it for you thus saving you time. It can also save your personal preferences, sites with customizable content, where you can choose what information and to show, also use persistent cookies. When used for the purposes named aboven a persisten cookie does no harm to your privacy. A persistent cookie can also be sent out by advertisers to identify your computer, not you personally, since cookies cannot dig into your computer for information. With this identification, sites which use this cookie can 'see' where you picked up the cookie and create a new one, thus enabling the advertiser to see which sites you visited (only those where the above named cookie was placed/loaded). Since no personal information will be sent the cookie is relatively harmless but, your privacy is still at risk, though a small one. Instead of being personally identified the advertiser only has a number/code to identify your computer.


Though cookies are relatively harmless through, for example, cross-site scripting (XSS) someone can 'hijack' your cookie and thus take over your browsing session.Ccookies store passwords, usernames etc. so in fact your' identity' can be stolen. A hacker can also inject code into a cookie on your harddrive through cross-site scripting, influencing your actions on the site the cookie is being used by.
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