Definition of tell
A tell is a sign that we decode in a player that gives us an indication of the nature of his hand. The tells can be multiple. Some players will have shaky hands when they have a very strong play, it's a tell! Some players will have a smirk on their face when they bluff, it's another tell. To avoid being read by their opponents, some people in casinos will wear sunglasses, a caquette and an hoodie. The more advanced players may, for example, feign certain gestures to mislead their opponents. For example, pretending to have a smirk on their face to make their opponent think they are bluffing.
The tells in online poker
The tells in online poker are less numerous since we can't see our opponents in the whites of our eyes, but they are still numerous. For example, a player who writes too much in the chat room is probably a novice or emotional player. We know he doesn't play many tables at the same time. A player who plays 15 tables at the same time and claims to have a HUD is probably a pro player.