For example, a player with a weak hand, hoping to bluff, may throw his chips into the pot forcefully and with a direct gaze at a player he hopes to discourage from calling.
Tells may be common to a class of players or unique to a single player. A player gains an advantage if she observes another player's tell, particularly if that action is unconscious and reliable. However, better players may fake tells, hoping to lead their opponents into costly traps when they rely on the false information. So the observing, creating, and evaluating of tells can add another level to the play of poker.
Mike Caro has published the most comprehensive information on tells; his Book of Tells (ISBN 0897461002) is now a standard reference on the subject.
David Mamet's 1987 movie House of Games includes an interesting discussion and visual reference to tells as an essential part of the plot. The 1998 movie Rounders contains an even more subtle use of strategy: at one point, "Mike" discovers a tell in his opponent (that he eats cookies in a particular way after he has bet a very strong hand), and after using it once, he reveals to the opponent that he has this tell; although this eliminates the usefulness of the tell itself, it upsets his opponent so much that it affects his later play.