As in most games, there exists a possibility of the players cheating. While playing online, you do not have to worry about someone making a false shuffle or peaking at your cards. What you have to worry about are players who play as a team, uses more than one computer, abuse the "all-in protection" or are using robots.
Players who have teamed up can gain a small edge by sitting down at the same table and giving each other information about their respective hands. This type of cheating is rare and does not have any significant effect in your wins or losses when you are playing at a full table. The fact is, many players who cheat in this fashion probably lose more or win less than they would have had they just focused on their own game. More dangerous are players who are cooperating to escalate pots, in which one of the players has a good hand, typically by raising each other to "boat in" the victim. However, this type of cheating is very easy for the poker room to detect with both automatic and manual reviews. This is another time when checking hand histories can be helpful for you as a player.
Players who uses two computers and two different identities while still playing at one table, is another related type of cheating, which is also easily detected by the poker room.
The all-in protection abuse cannot be abused for a long period of time before the poker room will take action. However, it unfortunately is impossible to completely protect users from this cheat as long as the protection is available for players. This is by far the most common form of cheating you will ever face.
The risk that someone would be able to hack into your computer while you are playing is extremely unlikely. If you still worry about someone hacking into your computer while playing you should install a firewall. Most ISPs provide firewalls as part of their standard offering.