Flash cookies are used by flash player to store user settings.But some websites use this track the user by storing their tacking data in flash local storage.Adobe has solved this threat from flash player 10.1 onwards
Make sure you have updated the flash plugin and delete the data from your previous settings.Almost all major websites are known to exploit this loophole.The cookies will remain in flash local storage if even the you use another browser.So practically all the online activities of user can be tracked.Please make sure you follow the update from adobe :)
Follow this link:
http://help.adobe.com/en_US/FlashPlayer/LSM/WS6aa5ec234ff3f285139dc56112e3786b68c-7fff.html#WS6aa5ec234ff3f285139dc56112e3786b68c-7ffa