Extracted 07SEP2011 from http://mashable.com/2011/09/02/supercookies-internet-privacy/
Recently, online properties like Hulu, MSN and Flixster have been caught using a tougher version of the common cookie. These “supercookies” (aka “Flash cookies” and “zombie cookies”) serve the same purpose as regular cookies by tracking user preferences and browsing histories. Unlike their popular cousins, however, this breed is difficult to detect and subsequently remove. These cookies secretly collect user data beyond the limitations of common industry practice, and thus raise serious privacy concerns.
Supercookies are similar to the standard browser cookies most folks are familiar with, but are stored in different locations on a user’s machine, for example, in a file used by a plug-in (Flash is the most common). This makes them harder to find and delete, especially since a browser’s built-in cookie detection process won’t remove them either. Furthermore, some supercookies have additional capabilities, like regenerating regular cookies to prevent their removal by the user.
To make matters worse, removing master supercookies is much more difficult. It requires the user to dig through the file system and delete them manually, an inconvenient task even for advanced users.