Cheating in poker is any behavior outside the rules of poker that is intended to give an unfair advantage to one or more players.
Minimal-skill methods
The easiest and most common types of cheating require no skill of manipulation, but rather merely the nerve. Such methods include shorting the pot, avoiding house fees, and peeking at other players' cards. It is very difficult to prove because when confronted, at least the first time, the cheat often calls the cheating an honest mistake.
One minimal-skill method that occurs in non-casino and casino games happens when a player who has folded appoints themselves the tender of the pot, stacking chips, counting them, and delivering them to the winning player. Check-chopping is when such a "helpful" player palms a chip. Odorless adhesive can be used for this purpose.
Another minimal-skill method is going south (also known as "ratholing"), where a player covertly removes a portion of their chips from play while remaining in the game, normally in order to preserve the winnings as profit, or prevent a major loss in "big bet" games.
Skilled methods
A cheat may hand-muck or palm one or more cards. When a cheat is "mucking" the cheat is cleverly hiding cards in their hand, to later switch their hand for. This may also be done with a confederate.
A skilled cheat can deal the second card, the bottom card, the second from bottom card, and the middle card. The idea is to cull, or to find the cards one needs, place them at the bottom, top, or any other place the cheat wants, then false deal them to oneself or one's confederate.
One sign of false dealing could be when a dealer grips the deck with the index finger in front of it or their pinky and pointing finger on both short sides of the deck while the other fingers support the deck while the cards are being beveled slightly. This is referred to as the "Mechanic's Grip". It not only allows better control of the cards, but provides cover by showing the back of the top card, and without moving the hand holding the deck.
A cheat can place certain cards in a position favorable to the card cheat. This is called "Stacking". Stacking is more often done than "False dealing" because it doesn't look suspicious. There are a couple of techniques for "Stacking" cards. The most famous are: Riffle Stacking and Overhand Stacking. By Riffle Stacking the cheat stacks the card(s) while doing a Riffle shuffle. This form of stacking is the most difficult to master and the most respected under the card sharps and magicians. The Overhand Stacking method takes little practice, and is more likely to be done in a situation with a cheat. The cheat does a (what looks like) normal Overhand Shuffle. But while the cheat is shuffling they keep track of the cards they want to stack, and with a little practice they can manage to put the exact number of cards in between the cards they want to stack to make the next round of dealing favorable for the cheat.
Even if a cheat deals themselves a powerful hand, they may not win much money if every other player has nothing, so often the cheat will stack two hands, with one player receiving a strong hand and the cheater getting an even stronger one. This is called a "double duke".
A slight advantage for a cheat can be to know what cards are placed both on the top of the deck as well as the bottom card, so that information can then later be used to bottom deal or second deal themselves that card. The looking at the top or bottom card without the other players knowing or seeing it is called "Glimpsing" or "Peeking". There are a lot of methods for reaching the same goal. A method that is used most is called the "Shiner". A Shiner is a reflective object (such as coffee, a lighter, a blade etc.) that is placed under the deck, so when the cheat is looking into the shiner the bottom card is exposed, and every card that is dealt over the shiner can easily be peeked by looking in the shiner.
One method of cheating that involves both great risk and great potential pay-off is the cold deck—so called because it has not been "warmed up" by play (and thus randomised). Such decks are usually pre-stacked, and are introduced either at the deal, after the real deck has been shuffled, or before the deal, where a card sharp will make a false shuffle using sleight of hand. The latter method may require collusion or a pass if the style of play or house rules call for a cut. The skill lies both in convincing other players that the shuffle is legitimate and in ensuring that other players receive hands that are good enough to entice them into play, but not too good to arouse suspicion.