A cookie is a text file sent by a web server to a web browser, and stored by the browser. The text file is then sent back to the server each time the browser requests a page from the server. This enables the web server to identify and track the web browser.
Most browsers allow you to refuse to accept cookies. (For example, in Internet Explorer you can refuse all cookies by clicking “Tools”, “Internet Options”, “Privacy”, and selecting “Block all cookies” using the sliding selector.) This will, however, have a negative impact upon the usability of many websites, including this one.