A tell in poker is an expression, mannerism or pattern of behaviour which would give an opposing player a clue as to the strength or make-up or a player's hand. Picking up on the tells that players give out is an integral weapon in the armoury of any great poker player and many articles (and even one very famous book) have been written on how to spot these fatal flaws in your opponents' play.
You would think that there is far less information you can glean about your adversaries when they’re playing online, instead of facing them across a real poker table and, of course, you’d be right. There are, however, some online giveaways which, if you can spot them, will give you a very real idea of the strength or intentions of your opponent.
Always remember that there may be some deception going on whenever you’re playing and that your opponent may be deliberately misleading you, by pretending to give off a tell. This is as big a possibility in online play as it is in a real bricks and mortar cardroom.
However, in low stakes online games you won’t go too far wrong by making note of the most common of all the giveaways.