In many card games, to cut the cards (or to cut the deck) is a procedure used just prior to the cards being dealt to the players.
A common procedure is that after the cards have been shuffled, the dealer sets the cards face-down on the table near the player designated to make the cut, typically the player to the dealer's right. That player initiates a cut of the deck by taking a contiguous range of cards off the top of the deck and placing it face-down on the table closer to the dealer; the dealer completes the cut by taking the original bottom portion of the deck and placing it on top of the just-moved cards. Once the cut is complete, the dealer picks up the deck, straightens or "squares" it, and deals the cards.
Rules of procedure or etiquette may vary concerning who makes the cut, the minimum or maximum number of cards which may be cut off the top, whether the dealer or the cutter restacks the cards, whether a cut card is employed, and whether a cut is mandatory.
The practice of cutting is primarily a method of reducing the likelihood of someone cheating by manipulating the order of cards to gain advantage. Even if the dealer (or the shuffler, if he is not the dealer) does not plan on cheating, cutting will prevent suspicions, thus many rules require it. Some players also consider the cut to be lucky.
The contiguous section may also be taken from the middle of the deck. This is called "Scarne's cut", though in some settings this is considered poor etiquette or against the rules. A cut involving a very small number of cards, such as taking only the top card (or some cards from the bottom) as a cut, is often acceptable according to rules. The same is true when a player takes every top card save for one on the cut.
During informal card games, the dealer is typically not required to offer the cut, and even if offered, the designated player can decline the request. On the other hand, any player may specifically request to cut the cards before they are dealt. If a cut is requested by a player, it must be granted by the dealer.
In formal player dealt settings, such as in a casino or during a tournament, an offer to cut the deck is mandatory and the designated player must perform the cut, generally by inserting a cut card (a plastic card about the size of a playing card, usually solid-colored) into the deck; the dealer then makes the actual cut at that point in the deck. When the dealer is not a player (i.e. a casino employee), the cut is mandatory and is usually performed by the dealer. In this instance, the deck is cut onto the aforementioned cut card, and the cut completed; this prevents players from seeing the bottom card of the deck.