Poker tells are unintentional, nonverbal cues that reveal a player's strength or weakness in a given hand.
In other words, you are communicating more information at the poker table than you may realize. Each player brings to the table their own combination of poker tells, usually unbeknownst to them. The best poker players are aware of their own poker tells and use this self-knowledge to their advantage (more on this in a moment).
Some poker tells are unique to the individual player (such as a player who absently fidgets with their eyeglasses when they're nervous or the player who incessantly shakes their leg when they've got a great hand). However, there are certain poker tells that are considered standard, as they've been observed in large numbers of poker players everywhere. Some of these classic poker tells include the following:
1. Players will often lean back in their seats and cross their arms when they are confident that they hold the winning hand. Conversely, a player hunched over their cards, elbows on the table, may be less secure of their chances.
2. It is widely believed that if, after an opponent places a bet you notice them staring at you, it means they are most likely holding a weak hand and hoping that you will fold. Conversely, a player who thinks they have the nuts (the best hand possible at that point in the game) will generally stare at the chips in the pot, waiting to claim them for him or herself.
3. A player with a strong hand usually bets, calls, or raises relatively quickly. If they take a long time deciding what to do, they are probably trying to talk themselves out of folding (probably their best option at that time).
So watch your opponents closely. You can learn to identify their poker tells by finding connections between their unconscious (or seemingly unconscious) behaviors and the strength or weakness of their hands.
Examples of behaviors to look for include: twitches (especially facial), standing up from their seat to play out the rest of the hand, smiling, banter (including asking questions of their opponents), constant blinking, indications of dry-mouth, furrowed eyebrows, playing with their chips, how they hold their arms/hands/fingers, how frequently or infrequently they sip their drinks. Remember though that any of these behaviors could indicate either strength or weakness depending on the player, so be sure to pay careful attention to how an opponent's chronic behaviors correspond with their cards.
Granted, in online poker it may be harder to identify poker tells. Key poker tells to look for in online play are - the amount of time a player takes before betting, calling, or raising (keeping in mind that different players have different internet connection speeds), and a player's use of banter (if the website has "chat" capabilities at their poker tables, which most do).
Now for the ever-important warning! A cunning player will sometimes strategically adopt one of these behaviors in a hand (acting/performing the tell) in the hopes that you will pick up on it and, as a result, believe the opposite of what is true (that their hand is weak when it's actually strong or vice-versa).
Incidentally, this tricky strategy is also found in the check-raise: when a player, first to bet, checks (suggesting weakness) then, when an opponent in the same round of betting places a bet, raises (revealing strength). But be careful, this too can be an elaborate form of bluffing.
It is therefore extremely useful to try to glean, as quickly as possible, an opponent's level of experience. That way you can better determine whether your opponent is genuinely unconscious of their poker tells, or simply trying to trick you.
The best advice I can give you here is to first learn your own poker tells and get a real handle on them. Otherwise, your own poker tells may be your undoing.