Poker is a card game played with a small group of players. Typically each player antes something (amount varies by game) just to get dealt cards and then puts bets into the pot (money placed in the middle). Players try to make the best 5 card hand they can using their own 2 cards and the five community cards. The player who bets the most money and has a high enough hand when everyone else folds wins the pot.
The first round of betting is called the preflop. This is initiated by a pair of mandatory bets called blinds put into the pot by the two players to their left. These bets give the players an incentive to play and help make the game more fun.
Once the preflop betting is over one more card is dealt face up. This is known as the flop. There is another round of betting. Players now compare their hands. A pair of matching cards, called a pair, is the best hand you can have. If you have two pairs then the rank of the higher pair is used to decide the winner.
After the flop there is a river and more betting takes place. At the end of each hand all cards are revealed and the person who has the highest ranked hand wins the pot. The runner up will usually win part of the pot as well. In some games the winning player is not allowed to show his hand, this is called a secret hand.