Poker hands are ranked in a hierarchy of specific combinations based on the probablility of that the hand will appear. A rare combination is ranked high while a common occurance like a pair is ranked at the bottom. The combinations are straight forward simple and intuitive such that anyone can easily start to play after a little practice. A full poker hand consists of 5 cards with some combinations using only a subset thereof.
Single cards are ranked from  low to high 2-3-4-5-6-7-8-9-10-J-Q-K-A.

Sample poker hands ranked from highest to lowest...

Straight Flush

Straight flush consists of 5 cards of the same suit (flush) with the added requirement that all the cards are consecutively ranked(straight)  A straight flush is the highest possible poker hand. The example shown here with the highest possible ranked cards to make the straight flush is known as a Royal Flush.

Four of a Kind

4 of a kind as the name implies consists of 4 cards with the same rank.  Not as rare as a straight flush but still relatively  few and far between.  

Full House

A full house  is achieved when you hold 3 cards of one rank combined with 2 cards of another rank. In this example we have Aces full of Kings.  (Also called a "boat")

Flush

A flush is 5 cards of the same suit.  Notice that unlike the straight flush example from above, this time, we only need to make 5 of the same suit without any restriction on the ordering of the cards. In this example, the highest card determines the value relative to any other flush. ie.. here we have an "ace high flush"  which is  the best flush you can make.

Straight

A straight is  5 cards of any suit in a consecutive order.  The lowest  value straight is a  A-2-3-4-5  and the highest value  straight  has  10-J-Q-K-A.  Note that the ace can serve as the lowest or highest card  but it does not "wrap around" meaning that  you cant make a straight with  Q-K-A-1-2.  Also the  A-2-3-4-5 straight is  the only exception where an ace is not the highest ranked card.

Three of a Kind

Three of  a kind is 3 cards of the same rank. Also referred to as "trips" or a "set"

Two Pair

2 cards of 1 rank plus 2 or another. You may have noticed that  rank combinations  is the essence of a poker hand. The suit is only relevant  in the  flush. Everything else is dependant on rank. So basically, it does not matter what the suit of a  combination is.  The only time you will pay any attention to the  suit of a  card is when you are dealing with a flush.

Pair

A pair is the lowest combination and the most common hand you will deal with in poker. Somebody somewhere has at least a pair of something. The most valuable pair is  AA and the lowest pair is 22.

High Card

If none of the above combinations is present, the best hand is simply the highest single card available. In this example we have a "high card ace"

Breaking a tie
In the case of a flush or a straight, the combo with the highest ranked card is the winner.  ie.. Ace high flush beats a king high flush
In the case of a full house, the player with the higher dominant (3 card) combination is ahead.  ie Aces full of twos beats kings full of Queens
If  the ranked cards are the same, the kicker (card not part of the hand) is used to break a tie.

If none of the above breaks a tie, the pot is split.