They're all hoaxes.
Some people even sell bogus cheating software, which is either an attempt at stealing your credit card information, or their software monitors your computer so it can do bad things (like steal names and passwords, or reveal your hole cards to others).
Because only limited information is sent to your computer during play there is simply nothing for the cheater software to decipher.
----------------------
Now, there is some illegal software that uses a group database to get information on other players. That is, this software watches millions of games and can tell you how often people raise pre-flop, how often they c-bet, how often they win at showdown, etc. So you have information on players that you did not gather. This is illegal.
Don't confuse this with legal software like Hold'Em Manager and PokerTracker 3, which record & display everything that people have done while you were at the table. These programs will also display the cards that a player mucked at showdown (which it retrieves from the hand history).
