What's an evercookie?
An evercookie is a persistent cookie that uses numerous clever techniques to avoid full deletion by the user. Think of it as a "super-cookie" that can be used to track your online activities even though you have taken steps to protect yourself, and you think you are protected against tracking.
To make matters worse, evercookies use something known as a local shared object (a Flash variable) to provide cross-browser tracking. This is another great reason to block Flash by default -- if you are a FireFox user, you really should be using FlashBlock, NoScript, or other techniques to avoid auto-loading Flash animations in your browser.