There is a lot of traffic that goes back and forth between your client and the poker site server, but let's look at a couple to illustrate how the traffic is processed. When you click the ante button, The GUI (Graphic User Interface), the monkeys in the GUI room passes a message through the slot to the Poker client monkeys. Player #4, at Table Hamburg, Antes. The client Puts this message in an envelope and addresses it to your poker server, table Hamburg, Player #4, and passes the envelope to the OS for dispatch to the poker server. The OS passes the envelopes to a special temporary room, which has been set up to communicate with the server. It contains the keys that will be used to secure the msg. The envelope is then placed in a lock box that can only be opened by the corresponding session room at your poker site. The locked, encrypted, container is then readdressed, to your poker site, from you, and passed to the network level. The lock box is now put into another container which may or may not be encrypted, addressed to the nearest, transfer point, an error check done and passed to the last room in your computer. This is the modem that sends the multi-wrapped container to your ISP.
Probably your ISP doesn't know the shortest route to the poker site, so it will forward the container to a central clearing house, where it is processed and sent on until it reaches a transfer point that knows the address of the poker site. Once it reaches the poker site ISP it is passed to the poker site servers.
At the poker site, the container is processed in reverse order through the same rooms as on your computer, at each room the ‘container' from the corresponding room is stripped off and the message is passed on. When it passes out of the Network Room to the session or encryption room, it is sent to another temporary room that is set up to process, decrypt, only messages from your computer. When you first sign on these temporary rooms are set up and keys to the lock box, encryption are exchanged. Not only are they different for each player, they are different each time you sign in to your poker client. Now the message is passed to another temporary room at table Hamburg, Player #4, and passed on to the game - Player #4 antes. Once the corresponding messages for all of the players have been received, the game deals the cards.
To deal the cards, the game passes messages back through the slot to the Player #4 room. "Player #4 was dealt, the 9 of hearts and the 3 of spades." The monkeys there address the message to player #4, Table Hamburg, Poker Client, on your computer and pass it back to the Session room where the message is locked in a lock box and passed back down through all the rooms until it is finally sent out on the virtual highway of the internet. It travels back through your ISP, to your computer up through the rooms where you see, player #4 dealt 9- 3 off suit.
If you look at this model, from the session room in your computer, to the session room on the poker site's server, there is no place where an external application can peek at your cards. Since the rooms between the session room and the actual table are temporary and only dedicated to you, there isn't an opportunity there for exploitation by an external application. The only place where a peeking opportunity occurs is inside the game client on the server or the client on your computer.
In both these cases it would require a significant code alteration within the poker client itself. Either the Code of the server client has been compromised or some one has gotten a remote viewer on your computer. The possibility of either of these is quite remote and does not involve some "cheater" program. Poker sites understand that security and fairness are the most important part of their service. They do everything possible to maintain the integrity of their software. Only an insider with access to the source code could make the required changes to the code. Putting a remote viewer on your personal computer would be much simpler. In fact, the simplest hack would be to put a key logger on your computer and steal your password and ID and then they could easily pilfer your account. Why spend time playing poker? They can simply transfer the money to their accounts. For that matter why bother with poker accounts at all? They have ready access to all of your bank accounts that you access online.
In conclusion, the weakest point in the whole system is your computer and specifically your ID and password. There are ways that the poker site can assist you, like separate log on IDs and table names, but your own computer and network security is the most significant protection for all of your accounts.