"Poker, like a lot of things in life, is all about strategy. - Poker Professional Daniel Negreanu in a Commercial for Poker Site FullTilt.NET
Tells are subconscious actions by a player that may give an observer person an insight into that player's hand. Mastering the psychology of poker is crucial to winning poker, and reading tells is one of the skills you need. When you first sit at a table, or even better, as you are watching a table to decide if it is where you want to play, pay attention to the players. This gives you time to notice tells, since it is easier to observe player actions when you are not in a hand. This is one of the reasons that, when you first sit at a table, you choose a seat where you will be waiting for the blind to come to you, rather than paying to start playing immediately.
The importance of player tells can not be understated, both your opponents' and your own. Mike "The Mad Genius of Poker" Caro and many others have made a living espousing the importance of player's tells for more than two decades. His most commonly quoted tell is "Weak is strong and strong is weak." In simple terms, if opponents, through their words or deeds, act as if they have a strong hand, they very often have a weak hand and vice versa. Since it is such a well known tell, many people try to reverse it, so beware of this tell when it is out of character for the opponent. Phil "The Poker Brat" Hellmuth, winner of ten WSOP bracelets, says that figuring out just two tells a tournament can make the difference between cashing out and busting out.
You need to know what many of the common tells represent and then attempt to see them in the players around the table. Check each of the other players' mannerisms to spot any obvious tells that may give you an advantage later. Another important aspect of knowing about tells is insuring that you yourself are not providing tells to your opponents. You can either stifle obvious tells, or more effectively, do the opposite. The best defense is to mix it up and, from time to time, "make strong mean strong." Although tells can be important information and a weapon needed in our poker arsenal, it is equally important to realize that tells are only one small part of the entire story and should never be taken in isolation or as an absolute truth.
It is just as important to not provide betting tells to your opponents. You do this in one of two ways. One way is to keep your bets the same size whether you are holding King-King or 5-7 offsuit. Also, take the same amount of time to bet the monster hand as you do when you fold. The other way is constantly changing the size of your bet. By making the bets different sizes for the same cards, you portray a confusing pattern to opponents. By being inconsistent, you do not allow your opponents to get a read on your style. Remember that the less information you provide your opponents through your own betting patterns, the better.