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. Tells can be multiple. Some players will have shaky hands when they have a very strong play, this is a tell! Some players will smirk when they bluff, that's another tell. To avoid being read by their opponents, some people in casinos will wear sunglasses, a cap and a hoodie. The more advanced players can, for example, fake certain gestures to mislead their opponent. For example, pretending to smirk to let their opponent think they are bluffing.
Online poker tells
There are fewer tells in online poker because you can't see your opponents in the whites of their eyes, but there are still many. For example, a player who writes too much in the chat room is probably a beginner or emotional player. We know that they don't play many tables at once. A player who plays 15 tables at a time and says he has a HUD is probably a pro player.