Poker hands are ranked based on the odds against drawing them. The rarer a hand, the stronger it is.
Some poker variants use alternate hands, but the rank of hands shown below (and at right) is standard. Hands that are tied in terms of rank are decided based on the highest card in the hand. In other words, a Pair of Kings beats a Pair of Tens, and a Straight 7-8-9-10-J beats a Straight 3-4-5-6-7.
The lowest possible hand is no hand at all -- no pairs, no Straight, nothing. This kind of hand is known by the highest card it contains, so K-7-5-3-2 of different suits would be "King High."
The lowest actual hand is One Pair, made of two cards of the same value and three unrelated cards.
Next is Two Pair: two cards of the same value, another two other cards of the same value, and one unrelated card.
Three of a Kind beats Two Pair: three cards with the same value and two unrelated cards. This hand is sometimes referred to as a Set or Trips.
Topping Three of a Kind is a Straight, made of five cards, any suit, in uninterrupted order of value. An Ace can be either the low card or the high card in a Straight, but you can't build a Straight between a King and a Two. In other words, A-2-3-4-5 is a Straight, and so is 10-J-Q-K-A, but Q-K-A-2-3 is not.
Just above the Straight is the Flush: five cards of the same suit, with any value.
Next is the Full House. A Full House is basically three of a kind plus a pair. A Full House made of three Queens and a pair of Sixes would be referred to as "Full-house, Queens over Sixes."
Four of a kind is a rare hand without wild cards in play. This hand consists of four cards of the same value, plus one unrelated card.
The strongest hand in poker is the Straight Flush, which is really a combination of a Straight and a Flush. This hand is made up of five cards of the same suit, in uninterrupted sequence.
The legendary Royal Flush is the rarest of all poker hands. Technically, it's just a Straight Flush with an Ace as the high card, but because it's an unbeatable hand, poker players usually consider the Royal Flush a hand in its own category. Specifically, a Royal Flush is five cards of the same suit in an uninterrupted sequence that leads up to the Ace: 10-J-Q-K-A.