Good Hold'em players can 'Read the Board', and they know what hands might be lurking out there at the table ready to show themselves and take the pot. They also know what hands can not be possible, and so they know not to worry defending against them. How can you tell what hands are possible, and what ones are not? Here are some simple guidelines to get you started.
Pairs on the Board - if there are a pair of cards on the board, someone may have a Four-of-a-Kind. If there are no pairs on the board, no one can have a Four-of-a-Kind. Pairs on the board also make a Full House possible. Both of these hands are very strong, so when the board pairs pay attention. Someone may be holding a monstor hand.

Three of a Suit - is what is necessary for a flush. Sometimes when the suited community cards are not particularly impressive, say 2-7-9 of clubs, it becomes easy to overlook the potential flush in your opponents hands.

Sequence Cards - the cards that make a straight possible. If the cards on the board only have two 'holes' in a string of five consecutive cards, someone may be holding those cards and just may have that straight. So, when cards like 9-J-Q show themselves on the board, beware that an opponent may be holding the 'missing' 10-K in their hand. Of course you know what cards are in your hand, and that can help you deduce what cards your opponents may have. Reading the Board - The board shows K♦-K¢¼-9¢¾-2¢À-6¢À - You have: A¢À-K¢À in your hand Is there a pair on the board? - Yes. Both a Four-of-a-Kind and a Full House are possible Are there three suited cards on the board? - No. A Flush is not possible. Are there three Sequence Cards on the board? - No. A straight is not possible. Can you use your cards to rule out possible hands? - Yes. Since you have one of the K, no one can have Four-of-a-Kind this hand. Conclusion - Because of the pair on the board, Full Houses are possible. Your opponent would need to have K¢¾ and either a 9,6, or 2 in their hand to complete the Full House. Or, they may have 2-2, 9-9, or 6-6 and complete the full house in that manner.

this article compliments of http://www.pokerprophet.com