There was a big poker scandal recently where someone was playing on a poker stream and getting fed hole cards in real-time instead of the normal 30-minute delay.
Collusion is prevalent and there are ways to mark cards which is why you should always get a new deck if you notice marks or creases.
Poker, being a game of statistics can be cheated in a variety of ways.
The main method would be to get more knowledge of the cards played to other players so you know the chance of being dealt cards that improve your hand.
For example in a live casino game Phil Ivey was accused of cheating as he identified cards or ranges of cards from the slight difference in patterns on the back of them.
Whether this was truly cheating is debatable as this was under the control of the casino.
A less debatable method of cheating is by colluding with other players.
You can gain a statistical edge if you know the cards held by one or more players.
It's easy to imagine how this can be done in live games by players signalling to one another.
Online poker has made this form of cheating easier as people can easily play on the same table without anyone knowing.