If you are concered about your privacy while browsing you probably know about cookies and handle them properly (delete them automatically when closing a browser session, block all cookies except whitelisted ones, ..).
Macromedia/Adobe introduced something similar to cookies called “Local Shared Object” in Flash6. Unfortunately browsers (at least Firefox) currently do not handle Flash cookies easily. If you tell your browser to clear its cookies they simply persist.
I was very surprised by the vast amount of flash cookies located on my system. Take a look yourself “~/.macromedia/Flash_Player/#SharedObjects/” for Linux, “~/Library/Preferences/Macromedia/Flash Player/#SharedObjects/” for Mac OS X or “%APPDATA%\Macromedia\Flash Player\#SharedObjects\” for Windows XP/Vista.
Two Flash cookies on my system are related to a browser game all others were not needed and I deleted them. You can configure your flash preferences at Adobe. If you want to deny all Flash cookies you need to set the amount of disk space that can be used to “None” at the “Global Storage Settings”. You will then be asked everytime a Flash object wants to set a cookies unless you also enable “Never ask again”.