First Off - What is a "poker tell"
A tell is an act or gesture that a poker player displays when trying to cover the strength of his/her hand. In live play at home games or casinos this could be acts such as smiling, talking a lot, covering their mouth with their hand or counting their chips. There are many different tells and each tell means something different about each player; the key is to figure out what they mean.
These are a little different for online poker as you cannot actually see the players. There are some common online tells that are known and observed by great players. Although this is a great guideline you should not solely follow these tells when playing. With that said, they will be right more often than not.
Insta-call - This is when the player instantly calls after you make a bet. This usually shows the player either has a draw hoping for a 5th card for the flush or straight, or the player is trying to show strength so you will stop betting.
Delayed bet - When an opponent delays betting for a longer period of time than normal - they will often hold a strong hand. They do this to either show weakness acting like they are contemplating betting or they are deciding how much to bet so you will call and they can get the pot bigger.
Delayed check - Often when a player delays before checking they will have a weak hand. They are trying to show strength, when they really just want to see a free card.
Insta-check - Most of the time this shows the player holds a weak hand. They probably have clicked the check or check/fold button because they are unhappy with their hand. Be careful though, as some players with very strong hands do this to induce betting.