If you are going to succeed at online poker you have to know when somebody is bluffing or actually has the nuts. Since you can't look them in the eyes, or watch how they shuffle their chips when you are online you have to look for other signs that they may be representing aces when they only have a pair of two's.
A tell is anything that a player does that gives you an idea of their hand strength. If you saw the movie Rounders with Matt Damon, KGB always twisted an Oreo when he had a weak hand, and Matt used it to beat KGB and get his money back. In a brick and mortar casino, a lot of players wear sunglasses to hide their eyes, which can be a very common physical tell. Online though, it's not as easy to see when someone has flopped a great hand or is trying to steal the pot.
Because online poker rooms let players decide how they are going to play before the turn comes to them, there is the option to automatically check, fold, or raise. Unless they are the big blind and instant check shows a weak hand and the player is hoping to improve on the turn or river.
The opposite of the Instant Check is the Instant Raise. This is a sign of a strong hand where the player is trying to increase the pot size. Unless you are sure you hold the best hand, it's a good idea to fold.
This tell is an odd one. A player will wait until the point that they are almost timed out before acting. It can represent weakness if they check or strength if they check. More often than not though, it irritates other players and forces some to go tilt.
If you are in a tournament long enough some players will reveal their hands by the amount they bet. An example would be someone who always bets 200 every time they get an ace. When you figure this tell out you can throw the other player off their game by telling them what they have.
Now that you know what to look for at the table, you can use these tells to your advantage. If you are strong but want to appear weak, throw in the occasional instant check to try and draw people in, then raise them all in. Better yet, wait as long as possible before checking on the river if you have the hand and then try and get all their chips