Bunco is a dice game, the object of which is to tally the most victories, or buncos, during four sets of play. There are six rounds to each set, corresponding to the six sides of the die. Traditionally, the game is designed for 12 players.

At the beginning of the game, four players are randomly chosen to sit at the head table. The remaining players are distributed among the remaining two tables. Each table is divided into two teams. One person on each team tallies the points. Along with the team score, a score is kept for each individual player as well.

The head table sets the pace of play which commences with the ringing of a bell. Play immediately begins as one player on each table rolls three dice attempting to roll “ones.” The target number corresponding to the current round of play. For instance, in the first round, each die that turns up a “one” in this case, scores one point. Thus, if two of the dice rolled came up as ones, two points would be tallied. Anytime three of the target number are rolled, for instance, three fours in round four, a bunco is achieved. The player rolling must call out "bunco" for his team to be awarded the associated 21 points. Only the individual who rolled the bunco can claim it on their individual score.

If a player rolls three of a kind of a number different than the number of the current round, she is awarded 5 points. As soon as one team at the head table reaches either 21 total points or scores a bunco, the head table rings the bell and the round ends for all tables. The round is won at each table by the team with the most points at the time the bell is rung. Awards are provided to the players with the highest scores at the end of four sets of play. 

http://www.wisegeek.com/what-is-bunco.htm