In friendly games it is common to be playing against someone you know well. Perhaps your wife may be playing at the game with the rest of the guys. Suddenly your luck turns for the worse. Subconsciously, you are less willing to take the money of the people you know or love. Perhaps one fellow has been getting bad hands all evening, and you know he has car payments to make, and this changes the game being played. The best advice is to leave friendship outside the poker game. Especially in tournament poker, soft-playing a friend is cheating all of the other players out of their chance to see you bust your friend, getting them closer to the prize money.
For this reason, there are laws in some U.S states saying that a husband and wife cannot play poker at the same table. Perhaps the easiest way to exploit such a situation is to argee to split the profits (after all, couples often have shared bank accounts). Even without any explicit collusion during the game, this reduces the variance of the team as a whole.
It should come as no surprise that two people sharing information about their hands enjoy a great advantage over the other players. If you don't believe this, deal out a few poker hands, but deal yourself two. The idea is that these players signal one another and only play the better of the two hands. Signals can take many forms, from the placement of the chips on the cards to morse code tappings on the table. The key ingredient in all signaling systems is the ability to be repeated unobtrusively. In order for this advantage to make money it has to be done many times without someone realizing it. In a game where people (hopefully) are always watching each other, this can prove problematic. When a cheat is signaling the value of his hand to his partner, he is also signaling the value of his hand to everyone at the table. The result of a system of signals being figured out is nothing short of finacial disaster. Some games are more susceptible to this kind of cheating than others: in Five-card stud and Lowball, for example, signalling the rank of just one card can give another player sufficient information to make many otherwise difficult decisions.
Should two people wanting to cheat be in close proximity, they might decide to hand-muck. That is, to switch hands or alter them in some way (though this particular form of cheating might be considered mechanics rather than collusion). A simple idea of this is to have two people sitting next to each other in a game of draw poker. While they receive two mediocre hands, they could switch certain cards between themselves in order to form a worthless hand and a winner. There are many sleight-of-hand methods to this. Hand-mucking is also a problem in blackjack.
Perhaps the most odious way of cheating with a partner is to have a weekly game at your house, agreeing with all your regular players that you split the profit from cheating a single player. This hot-seat game invites a new player every week, only to play against six players all working together. The mechanics are the same, players signal their hands, then play proceeds as to drive the hot-seat out, or to put all his money in the pot.
If you are at a poker game and you detect that your opponents are cheating, but aren't very good at it, you can use this information to your advantage. You may be better off exploiting their inept cheating than leaving or turning them in. Frank Wallace wrote a book on this, in which he coined the term neocheating. Whether or not this should also be considered cheating is an interesting question of ethics.
Another source for reliable information on card cheating is CARDSHARK Online at http://cardshark.us/