A tell is anything that a player says or does that can provide an observing opponent insight into what cards players are holding and what they are thinking. The top ranking poker players in the world hardly rely only on their skill and the luck of the draw. They learn to collect subtle clues from the other players to arm them with a proper view of the strength or weakness of opponents.
Common tells include licking of the lips, the way players hold their cards or shift their cards, shiftiness in the way they see other players, or the way they idly shuffle their chips. You may perceive two kinds of tells and both signify the level of anxiety, stress, or confidence about the cards in the player's hand. The first are more obvious movements that come out as leaning forwards or backwards in their chair, shifting cards, and changing their betting patterns. These are important tells that professional players work very hard to control and hide. The other type of tells are quite individual to the player and much more difficult to hide as they are subconscious eccentricities. They may include holding the chin, pulling an ear, rubbing their hands, and are important to watch for since they are just as informative.
The main thing about tells is that it is far from enough simply to notice the behavior. You have also to link the tells you have seen with the pattern of behavior; to note down when they have bluffed and when they have come out with a very strong hand. The best way to make a note of this is to watch every player when they first lift their cards, and when they are about to make bets. Carefully look for any apparent signs and then connect these with the results of that hand. It requires that you keep a lot more in mind through each hand, but if you can, you will have a large lead over all your opponents