It's a two-player card game, most recognizable by a pegboard used for scoring.
Basic idea (not the precise rules):
For every round, each player gets 6
cards and discards two. The four discarded
cards become the "crib;" the crib is not viewed until all cards have been played for the round. The top card of the deck is exposed. Players alternate placing cards, nondealer first, and score points on various occurences (including total of cards = 15; closest to 31; runs of 3 or more; pairs). Players then score their hands for various combinations, including totals of 15, multiples (pairs+), flushes, and runs; these combinations are made in conjunction with the upcard. Dealer gets the benefit of making combinations using the crib. Winner is first to a predetermined number of points, usually 121.
There are a handful of details missing. More complete rules:
http://www.mastersgames.com/rules/cribbage-rules.htm [ November 17, 2004: Message edited by: Jeff Langr ]