The Importance of Tells
The biggest thing that sets apart beginner poker players with those who are experienced in the game is the ability to read one's opponents while keeping other players from reading them. Tells are the mannerisms that each person has that gives away their feelings and thoughts at any given time-- not just during a game of poker! Tells are the best way to have some measure of control in poker, no matter what variant you're playing. Some people think that players only have tells in live games, but this isn't true at all-- there are countless clues that will help you decode the actions of other players, and while some are specific to certain types of games (community card games, for instance) and others require seeing a person face-to-face, there's no shortage of ways to determine what's going on in an opponent's head when playing poker online.
Learning to Spot an Opponent's Tells
Tells vary from person to person, but there are some indicators that are pretty much universal to all people, unless an individual has spent a lot of time and energy working to conceal their natural reactions. If you learn to spot these behaviors, you'll be one step ahead of the competition, and you can play poker with more confidence, which will in turn greatly improve your game (you'll also have the fortunate side effect of knowing whether or not your opponent made the flop, which can keep you from losing money!). Keep in mind that some experienced players will throw out fake tells just to be able to throw you off track at the most opportune moment. Despite this, knowing the ins and outs of tells can only improve your poker game.
The most common tells that players (and people in general) have are:
Checking one's cards
A player checking his or her cards is looking to see if they've made a draw-- or have the potential to turn a weak or mediocre hand into a decent one with a future draw. This is the biggest tell that people have that says "I don't have a very good hand, so I have to check to see if it's worth staying in this."
Checking one's chips
A player checking chips is probably trying to determine just how much they can get away with betting-- this is generally a sign of a strong hand.
People's hands shake when they're nervous or excited-- a byproduct of an adrenaline rush. Watch out for a player whose hands are trembling as they bet or wait for the other players in a round-- this player probably has a great hand and may even be sitting on the nut hand.
Bet size/ Betting patterns
One of the best and most often overlooked ways that you can tell what a player is thinking or feeling is to watch the size of his or her bets. Most players have a small range of bets that they make, and anything outside of the norm should raise a red flag with you. If you watch your opponents and keep track of how they bet and when, you should get a good idea of why they bet the way that they do. Since the majority of players follow a pretty simple "If you have X, bet Y" kind of formula that they follow, if they don't change the pace and mix things up, you can crack this code and know exactly what their hands are as you play.
Speed of action
With rookie players, the faster someone makes their action, the stronger the hand, and the longer that it takes them, the weaker the hand. This is clearly not always the case, and it's probably the area where online players are most conscious about their tells. Some players will actually time themselves to make sure that they always make a decision in precisely the same amount of time, regardless of the strength of the hand, but new players don't know any better. If someone takes a long time to decide what to do, they're probably trying to figure out the pot odds to determine whether or not to hold on to their draw hand. If someone re-raises instantly, it probably means that they're sitting on a strong hand-- maybe one they believe to be the nuts.
Prolonged eye contact
Contrary to what most people believe, a person who is lying is more likely to make eye contact than one who is telling the truth. Because we're conditioned to associate downcast eyes with lying and direct eye contact with the truth, people who are lying will often make eye contact as a way to counteract this, leading to an overwhelming majority of people who are unusually aggressive with their eye contact when they lie.
Rate of respiration
Novice players will hold their breath when bluffing (it's normal for people to stop breathing when they're waiting for an answer for something about which they're apprehensive), and players who have a strong hand may have problems keeping their breathing slow and even, as the adrenaline of having what you know is a good hand can cause the heart to race and breathing to become faster and more shallow.
Shrugging is a sign of lying, both in the world of poker and without. It's not just a mannerism-- it's an action that's universal, for the most part. When people are unsure of something or actively lying about it, they'll shrug or do a half-shrug, so be sure to watch an opponent's shoulders when you're trying to determine if they're bluffing.
Touching one's face
Like shrugging, people touch their faces when they're nervous or trying to hide something, so this is an action that you see a lot when people are lying or bluffing. The face and hair are home to some of humankind's most common unconscious mannerisms-- twirling the hair, rubbing the chin, covering the mouth-- and they all mean different variations on the theme of discomfort. When someone covers his mouth, it means that he doesn't completely believe what he's saying (lying), and rubbing one's nose is a physiological response to stress (like when one is lying). Rubbing the chin or pressing on the nose can indicate that someone is thinking deeply, so if someone does this in a game, they're probably deciding whether or not the hand they have is good enough.
As a general rule (obviously there are going to be exceptions), players who are fast and loose will have messy chip stacks, while careful plodders and thinkers have neat stacks. When you sit down at a table, make a mental note about the chip stacks of the people around you, as it may say something about the way that they play.
Players who are worried that they might seem too excited about their hand will often default to what they think is the ideal poker face-- one that seems completely disinterested in the game and the action and especially bored with the cards in their hand. Here's a simple rule: poker players are never uninterested in what they're holding in their hand. If they really weren't interested, they would have folded, so someone who's holding onto cards and actively pretending not to care about them is trying to convince you that he doesn't have a good hand-- which probably means he does.
While facial expression is the first thing that people work on controlling when playing poker-- everyone tries to obtain a poker face-- most people focus on their mouth to keep from smiling and frowning, but this doesn't mean that you can't find indicators on their faces. Watch the corners of the mouth-- for just a fraction of a second when a player looks at his hand, you may see the corners of his mouth move down-- a sign of frustration and discontent. Also watch a player's eyebrows, as they tend to go up when a player is surprised.
Body position and posture
Players can give a lot away with the way that they use their bodies. When a player leans forward in his or her seat, it indicates not strength, but aggression-- this person is either sure that he has the nut hand, or he's bluffing, but either way, he's issuing a dare (generally, this is more common bluffing behavior, unless this person is a rookie). Players who are discouraged, either by their hand or the course of the game, may have bad posture and slouch in their seats, while players who finally get a hand worth playing may sit up a little straighter-- all without realizing it.
Discovering and Hiding Your Own Tells
There are two solid, surefire ways to learn your own poker tells when playing a game. While lots of people will tell you that you should play in front of a mirror, it's likely that you'll feel more self-conscious and aware of your actions and change them, therefore not giving a really accurate reading of what you do. If you're playing live games, the best ways to tell what habits and mannerisms are giving you away is to either record the games that you play and watch them after the game is over, or play with a friend who is truly invested in helping you become a better player-- one who doesn't potentially benefit from imperfections in your game!