For any game of Poker, tells are a key element of analyzing betting. Analyzing betting patterns is an great part of playing winning online poker: call or raise, speed of bet, sizing of bets in pot limit or no limit. A huge percentage of online players now are regulars. Playing very many games means they get into rhythms or habits. Their normal rhythms are exploitable, but any departures from the norm represent the key moments to focus on. You don't want to call someone's all-in bet when they move away because they have the nuts, but you do want to call when they stray because they have a busted draw in a key pot... and you clearly do not want to be clueless about the very fact that they have swerved!
The Rant - Many online poker tells are a consequence of bad players telling you that they are about to play even worse than normal. The most apparent of these announcements is The Rant. The fact about online is bullies and loudmouths can't glare at, roll their eyes or do some other belittling physical action. But via the chatbox they can very well insult their opponents. They cant declare "change the deck" but they can rant about software being rigged against their advanced level play.
Going to the basics, a player who goes on a rant about dim opponents or rigged software might just as well paint themselves in purple with ten inch letters: "I am on tilt". Also clearly horrible opponents, ranters are the main players to target in any game, even more so than noticeable multi-tablers. You want to play against people on tilt, but you also want to be sure you don't take them off tilt by playing a feeble hand at them that assists them to calm down. Go ahead and goad rude players in the chatbox. These are nearly always players who think they are far ahead than they are, and play considerably worse when losing than when winning. They will often make the game. Value the ranters. To a large degree, winning poker is about beating people who act stupidly. Ranters simply top of the list.