The object of the game is to be the first person to score at least 121 points. Points are mainly scored for runs (groups of three or more sequential cards, regardless of suit); pairs, triples and quadruples (cards of same rank); flushes (all cards in your hand the same suit, can include the starter); combinations of cards that add up to 15; and a jack in your hand whose suit matches that of the starter. It is generally played by two people, but can be played by three (as individuals), four (as two two-person teams), or six (as three two-person teams).
The deal rotates with each hand. This is important because the crib gives the dealer an advantage (especially in five-card play). If at any point in a hand a player "pegs out" (reaches the winning score of 121), then the game ends and that player wins. A notable feature of cribbage is that as soon as a player pegs out, the game is over; this can happen during the play of cards or while the hands are being scored. Some cribbage boards are marked with an "S" in place of "90" because a losing player is said to be "skunked" if, when the game ends, that player has fewer than 90 points. A player who loses with fewer than 60 points is said to be "double skunked" or "lurched". A player who loses with 120 points is sometimes said to be "left in the stink hole".
The dealer shuffles the deck and deals six cards (there are different requirements for variations of the game). Each player then discards two cards face-down to form the crib (there are different requirements for variations of the game), which will be used by the dealer. The player to the dealer's left cuts the deck and the dealer turns up the top card, called the "starter", and places it on top of the deck, face up. If the card is a Jack, the dealer scores two points for "his heels", also known as "his nibs". This latter term is occasionally confused with "his nobs", which is more correctly used for a Jack of the same suit as the starter, scored during the show part of the game.
Aces are always low and have a counting value of 1. Face cards always have a counting value of 10 but retain their customary rank for runs, thus 9,10,J,Q,K is a valid run of 5. Because an Ace is always a low card, it forms a run only in combination with a 2 and a 3, not with a Queen and a King. All other cards are worth their face value.
Starting with the non-dealing player, each player plays a card in turn, stating the total value of the cards played resulting from the latest card played (ex. the non-dealer plays a 7 and says "7", the the dealer plays a 10 and says "17", etc.). The total can not exceed 31. If a player cannot play a card without bringing the total over 31, the player says "Go" and the other player(s) must play any cards that keep the total at 31 or less; the last player to play a card before the count is reset to zero pegs 1 point for the "go" (if some cards remain unplayed) or for "last card" (if no cards remain unplayed). If said player tallies exactly 31 points, another point for "31" is earned, making two points in total. The count then resets to 0, and the first player having unplayed cards to the left of the player who played last leads the next card.
During the play, players peg points as follows:
After the play, each player in turn in the order of play and ending with the dealer, tallies the points in their hand, including the starter card, and "pegs" (counts) the amount. When it comes to the dealer, the hand is scored first, followed by the crib hand. In a standard, two-handed game, the hands are scored in order: non-dealer's hand, dealer's hand and dealer's crib last. In general, the hands are scored starting with the player to the dealer's left, then continuing clockwise to the dealer, whose hand is counted first and the crib last. The order in which this is done is important as a player who scores first may peg out and thus win the game even though another player's tally would exceed that score.
Points are scored as follows:
Cards may be used in several different combinations in the same deal, thus a jack may score for his nobs, in a flush, a run, as a 15 (in combination with a 5) and as a pair.
The score is traditionally stated aloud as shown, though players may simply declare the score, particularly with low-scoring hands.
After all players have counted their hands, the dealer tallies the points in the crib. This is counted in the same manner as the other hands, except that a flush is scored only if the starter card is also of the same suit.
This material is provided courtesy of Wikipedia's Cribbage entry, and is released under the GNU Free Documentation License.