Go fish is a simple card exchange game of the quartet group, usually played by children. The basic objective of the game is to collect the most “sets” or “books” of cards before one of players runs out of cards or the draw pile runs out. Many popular variants of this card game have been devised, including Authors and Happy Families.
Go fish uses a standard 52 card deck and is usually played with 2-5 players. A dealer is chosen at random, and passes out 5 cards to each player. The rest of the deck is placed in the center of the players and is used as a stock. The player at the left of the dealer starts, and play continues in a clockwise rotation.
During a players turn, he may ask any other one player at the table if they have a particular rank that they currently hold in their hand. When asking for cards, he picks a player of his choosing and says, “Do you have any [insert rank here]”. If the other player does have any cards of that rank, he must give all cards of that rank that he has to the player who asked for them. The player who received the cards is then allowed to repeat the asking process. If the player that he asks does not have the rank that he asked for, then the other player says “Go Fish.” The asking player then draws a card from the stock, and his turn ends. However, if the card that he draws is the particular card needed to complete the rank he was just asking for, he sets down the book and his turn starts over again   
Whenever a player has four of any rank in his hand, the player must immediately set the cards face up in front of him. Four of any rank is known as a “book”. When any player has an empty hand, or all cards in the stock have been drawn, the game ends. Each player then counts up how many books he or she has. The player with the most books wins   
A common strategy to quickly gain pairs is to keep track of what people have asked for which ranks. For example, if someone asks for sixes, and you later draw a six, you can assume that the other player still has sixes in his hand. Because of this, you can ask for sixes on your next turn and be given the cards with absolute certainty (unless someone else asks for sixes first).
Because the game is so heavily dependent on the honor system, several illegitimate strategies can be developed by simply not handing over cards. For example, if a player were to have a three in his hand and someone were to ask him for threes, he could say “Go Fish” and ask for threes from the original player a few turns later. Because players cannot tell what other players draw from the stock, such tactics are virtually impossible to detect.
This game is similar to Go Fish, with two major changes: all of the cards in the deck are dealt equally to the players at the beginning of the game, and when a player asks for a card that the other player does not have, the turn passes to the player they asked. Unlike Go Fish, the game ends only when all cards have been formed into books.
The British version of the game uses 11 ranks of 4 cards each, resulting in a 44 card deck instead of the normal 52. At the beginning of the game, all players contribute to a pool, which is then divided into two halves. The players do two games, one where people ask for entire suits, and the other where people ask for specific cards (such as asking for the “King of Clubs” instead of just “Kings”). The winner of the first game gets one half of the pool, and the winner of the second game gets the other half.
http://uk.games.yahoo.com/online-games/card/games_gofish.html -Free online Go Fish program