Poker is a card game where players put an amount of money in the pot (the pot varies by the game). They then receive cards and place a bet on those cards. When all players are done betting the cards are flipped over and the best hand wins the pot. The highest hand can be a pair of the same cards or a flush.
The best hand is the Royal Flush which consists of a 10, Jack, Queen, King and Ace of the same suit. There are also four of a kind (four cards of the same rank) and a straight. A full house is three of a kind and a straight flush is five consecutive cards, not in order but the same suits (clubs, diamonds, hearts or spades).
You can also bluff in poker and it is important to do so with care as you will probably make some mistakes early on. If you are a beginner it is best to focus on relative hand strength rather than attempting complex bluffs.
Position is also very important in poker. The player in the first position (EP) should be very tight and open with strong hands only. The player to his left (MP) should play a little looser and consider raising preflop if they have a good hand. The last player to act (CO) has the most information and should be able to make more accurate value bets.
It is also very important to leave your ego at the door when playing poker, especially at the lower stakes. It will cost you a lot of money in the long run if you continue to battle against players who are better than you.