Collusion

Collusion is two or more players acting with a secret, common strategy. It is considered to be cheating, and is believed to be the most widely prevalent form thereof in online poker.

Forms
First and foremost, colluding players can benefit from knowing their partner's hand. This might induce them avoid playing certain hands based on such inside information. Consider for example if you hold a pair of tens prior to the flop. Under many circumstances you would call or even raise with such a hand, but if you somehow knew that another player held something like ten-three offsuit then you would know your odds of winning the hand to be cut by something in the neighborhood of 40%, that being the case you'd probably fold. Collusion allows cheaters to benefit from such knowledge and unfairly denies opponents a fair chance to win money from them in the process.

Some common forms of collusion are: soft play, that is, failing to bet or raise in a situation that would normally merit it, because you don't want to cost your partner money; whipsawing, where partners raise and reraise each other to trap players in between; chip dumping, where a cheater will deliberately lose to a partner; and signalling, or trading information between partners via signals of some sort, like arranging their chips in a certain manner.

Online specific
Simple collusion in online poker is relatively easy and much more difficult to immediately spot if executed well. Cheaters can engage in telephone calls or instant messaging, discussing their cards, since nobody can see them. Sometimes one person be using two or more computers to play multiple hands at the same table under different aliases (since many broadband plans offer customers multiple IP addresses, this can conveniently and cheaply be done without the likelihood of immediate detection). Such tactics can give cheaters an advantage that is difficult to work against. However, online poker cardrooms keep records of every hand played, and collusion can often be detected by finding any of several detectable patterns (such as folding good hands to a small bet, as it is known that another player has a better hand). Users who frequently sit at the same tables will be flagged by poker rooms and their play will be closely monitored. Often, such users will be warned they have been flagged, in an effort to deter collusion.

Checking down
In a poker tournament, when one player is all in and two other players are active in the pot, it is common for the two players with chips left to "check it down", or check on each round of betting through the end of the hand. Unless they explicitly communicate an agreement about checking it down, this is not collusion.