There are still people who refuse to play online because they are afraid of hackers. And indeed, there are dodgy websites that claim that they can read all the cards in the game or see the turn and river before they are actually dealt. Don’t be surprised by those claims. Most casinos have taken very professional safety measures that are impossible to avoid.
The most common method of online cheating is collusion, meaning that two or more players inform each other about their cards. Your online opponents might be sitting next to each other on a couch with their laptops. Fortunately collusion requires skill and practice and most players are unable to optimally benefit. Online poker rooms should not only look at the obvious signs of collusion, but also at the underlying evidence. Conspiring players often play at high-limit tables and quickly run off with their winnings.
Please not that collusion is quite unusual. Therefore you should always be careful with accusing other players. If you suspect dishonesty, then you should alert customer service or else thus leave the table and play at another. In most casino’s you will have plenty of choice.