How to Win
Be the first player to score 121 points.
Play Against Other People
Cribbage can be played against one opponent. By default, you play a game against the computer. For more information about how to play against other people, please read

Order of Play
At the start of each game, each player will draw a card to determine who will deal first. The low card deals first.
A game consists of several hands. For each hand, the dealer deals each player 6 cards. Four of these cards will make up your hand and two of them will be placed in the crib.

 The hand composed of the cards in the crib belongs to the dealer and is scored by the dealer. The players alternate dealing for each hand.

(the non-dealer, the dealer, and the crib).
 If the upcard is a Jack, Heels is scored for the dealer.
After the upcard has been revealed, play begins with the person who is not the dealer.

The players will alternate playing cards to the table, trying to get the total as close to 31 as possible.
When the total nears 31 and a player cannot play a card to keep the total at 31 or less, that player must call Go.
The opposing player is awarded 1 point for Go.
After Go has been called, a player must continue to play cards that bring the total closer to 31, if possible.

 If neither player can play, the count returns to 0 and play begins again.

Pairs, runs, scoring 15, and scoring 31 are all worth points and are scored immediately.

Once all the cards from both players are on the table, the game enters the scoring phase. The person who is not the dealer always scores first. Then, the dealer's hand is scored and the crib is scored.

The game ends immediately as soon as 1 player's score reaches or exceeds 121. This can occur during any portion of play.

Scoring during the play
A player who makes any of the following scores during the play pegs them immediately.
15:
If you play a card which brings the total to 15 you score two points ("Fifteen two")
31:
As mentioned above if you play a card which brings the total to exactly 31 you score 2 points.
Pair:
If you play a card of the same rank as the previous card (e.g. a king after a king) you score 2 points for a pair. Note that (for example) a 10 and a queen do NOT make a pair even though they are both worth 10 points.
Pair Royal:
If immediately after a pair a third card of the same rank is played, the player of the third card scores 6 for "pair royal".
Double Pair Royal:
Four cards of the same rank, played in immediate succession. The player of the fourth card scores 12.
Run:
A "run" or "sequence" is a set of 3 or more cards of consecutive ranks (irrespective of suit) - such as 9-10-jack or 2-3-4-5. Note that ace is low so for example ace-king-queen is not a run. The player of a card which completes a run scores for the run; the score is equal to the number of cards in the run. The cards to not have to be played in order, but no other cards must intervene.
Example: cards are played in the following order: 4-2-3-5-6. The player of the 3 scores 3 for a run, then the player of the 5 scores 4, and the player of the 6 scores 5.
Another example: 4-2-3-4-3. The player of the first 3 scores 3 for the run 4-2-3. Then the player of the second 4 score 3 for the run 2-3-4. The player of the second 3 scores nothing because the 3 does not complete a run.

Last Card:
If neither player manages to make the total exactly 31, whoever played the last card scores 1 point.

The Show
Players now score for combinations of cards held in hand. First the non-dealer's hand is exposed, and scored. The start card also counts as part of the hand when scoring combinations. All valid scores from the following list are counted.
15:
Any combination of cards adding up to 15  scores 2 points. For example king, jack, five, five would count 8 points (four fifteens as the king and the jack can each be paired with either five). You would say "Fifteen two, fifteen four, fifteen six, fifteen eight).

Pair:
A pair of cards of the same rank score 2 points. Three cards of the same rank contain 3 different pairs and thus score a total of 6 points for "pair Royal". Four of a kind contain 6 pairs and so score 12 points.

Run:
Three cards of consecutive rank (irrespective of suit), such as ace-2-3, score 3 points for a run. A hand such as 6-7-7-8 contains two runs of 3 (as well as two fifteens and a pair) and so would score 12 altogether. A run of four cards, such as 9-10-J-Q scores 4 points. This is slightly illogical - you might expect it to score 6 because it contains two runs of 3, but it doesn't. The runs of 3 within it don't count -you just get 4.

Flush:
If all three cards of the hand are the same suit, 3 points are scored for flush. If the start card is the same suit as well, the flush is worth 4 points. There is no score for having 2 hand cards and the starter all the same suit. Note also that there is no score for flush during the play - it only counts in the show.

One For His Nob:
If the hand contains the jack of the same suit as the start card, score 1 extra point.
Note that when scoring a hand, the same card may be counted and scored as part of several different combinations. For example if your hand is 7 8 8 and the start card is a 9 you score "fifteen 2, fifteen 4, and a pair is 6, and a run is 9 and a run is 12" - 12 points to peg, with each of your 8s forming part of a fifteen, a pair and a run.
After non-dealer's hand has been shown and the score pegged, dealer's hand is shown, scored and pegged in the same way. Finally the dealer exposes the four cards of the crib and scores them with the start card. The scoring is the same as for the players' hands except that: