What Are Cookies?

• Cookies are small files containing plain text stored by Web browsers allowing information to persist between pages. Each cookie is stored until either the expiry date is reached or the file is deleted. Since cookies are limited in size to 4 kilobytes (KB), they usually just contain a unique identifier such as "id=123456789." The website uses this information to identify you and display specific content, such as the contents of your shopping basket on an e-commerce site, which is stored on the Web server. Many websites are customizable to some degree, allowing you to select a language preference or theme defining the site's appearance. The information stored in the cookie allows the site to display your custom settings whenever you visit.
  
  

How Do Cookies Work?

• When you visit a website, it sends cookie content to be set, which your browser writes to a file on your hard disk. When your browser requests a page, it also sends the contents of the cookie. The website then uses the unique id to retrieve your personal data. Cookies are fundamental to the operation of most websites; without them e-commerce sites, forums, online banking and any other site requiring a login could not function.
  
  

Are Cookies Harmful?

• Since cookies store plain text, they cannot store programs or executable files and are completely safe. The issue is generally one of privacy, since no one likes to imagine a third party "tracking" them. The original purpose of cookies -- allowing a site to maintain information between pages -- is harmless in and of itself. On an e-commerce website, for example, the site can only access information you provide, such as your name and address. Websites can only read from their own cookies, not those set by other sites. To see information stored in a cookie, open it in a standard text editor. You may need to change your computer settings to allow you to view hidden files before opening cookies. Your browser's help feature can tell you where cookies reside.
  
  

Privacy Issues

• The privacy issues come from advertising networks, which pay websites in return for adding their code to the site. This code uses a cookie to display targeted advertisements and track sites you visit. Although ad networks can only track sites within their network, these are wide enough to build a profile on your browsing habits and background. If one site provides your personal information from a purchase, real world information such as your name and address could be stored against this profile and sold to third parties. Similar practices also occur offline, however. Supermarkets use loyalty cards to build a profile based on your purchases. Scanning your card to add points allows them to keep a list of everything you've ever bought, linked to your name and address to send you targeted marketing. These practices are so widespread many people no longer give them a second thought.
  
  

Managing Cookies

• Due to the privacy issues surrounding tracking cookies, browsers contain comprehensive controls regarding how they're handled. You can block sites from setting cookies and allow them from trusted sites. You can automatically delete them after a certain period or each time you close the browser, which prevents long-term cookies. Anti-viruses, firewalls and privacy protection programs also allow control over cookies, automatically blocking or deleting those from certain ad networks. Many browsers have special privacy modes that wipe out all traces of your browsing history when you exit. One disadvantage to this is needing to log in again to websites using custom settings.