Texas Hold'em
The basic principle behind Texas Hold'em Poker is that each player gets only two cards (which only he can see). The winner is the one of the remaining players at the end who can put together the best five-card poker hand as a combination of his two cards and five open cards (so-called "community cards") on the table.
In this example, Player A has a straight with his Jack and the 8, 9, 10 and Queen from the community cards. This beats the two pairs which Player B can form with his 3 and 9 together with the 3, 9 and one more community card.
But it is a long way to the end, with several opportunities to bet money or drop out of the game.
Not all cards are dealt at once. First each player just gets his own two cards, and everybody has a chance to bet money according to what they expect they will be able to form with the cards later. The procedure is known as a round of betting. Afterwards, the first three community cards are dealt. The players now know a little more about how useful their cards might be and they get another betting chance. After this round, the fourth community card is dealt, followed by another round of betting, then the fifth community card -- and finally the last betting round. When this is over, the players may reveal their cards and the winner is determined as explained above.
Betting is always done in clockwise order. The first person to act is the one sitting on the left-hand side of the player who deals the cards. Consequently, the dealer will be the one who gets to wait the longest before acting. Having seen what all the other players did, the dealer has access to more information and is in a better position than the others. To make things fair, the players take turns to deal the cards. After each finished hand, the dealer position is moved one seat clockwise.
The first person to act has three choices: he can bet, fold (i.e. give up) or check, which is the same as doing nothing and just passing the opportunity to the next player.
After the initial bet, if there is one, the rest of the players in turn have the corresponding three choices to raise the bet, fold or call, which means matching the bet of the previous players.
A round of betting is finished when all the remaining players have acted at least once and contributed the same amount to the pot. (To keep rounds from going on indefinitely, there can be at most three raises per round.)
The size of bets or raises is determined by the rules of the game rather than by the individual players. During the first two rounds, all bets and raises are equal to a number called the minimum bet, and during the last two, bets and raises equal the maximum bet. Typically, the maximum figure is twice the minimum and a Hold'em Poker table where, for example, the minimum is $3 and the maximum $6 is referred to as a $3-6 table.
A special case is when a game, or hand, starts. The two nearest players on the dealer's left-hand side have to place mandatory bets equal to half the minimum bet and the whole minimum bet respectively. Those bets are known as blinds due to the fact that the players have not yet seen their cards. This is to ensure that there is always something in the pot. Afterwards, the players' individual cards are dealt and the the first betting round picks up from that point.