Collusion: Think of 2 people chatting via aim or on the phone telling each other what cards they have. Then betting or raising to map larger pots. The player with a good hand now has a chance to win a much larger pot and the player with the bad hand just folds. Sure that could lose to a better hand, but if done right they could walk away with alot of your money.
Poker Bots: Here are some of the bot programs I have found. I do not recomend that you use a bot, infact I implore you not to, but you should know that they are being used and what they can do.
WinHoldem, Holdem Memory, PokerBot plus, and PokerBot Pro.