When you are hacked, your computer has been infiltrated by someone. You may or may not know them. Either way, someone has accessed your account/system without your knowledge. That is being hacked.
To NCSOFT's knowledge, there is no way your computer security can be compromised through our game clients.
However, you must beware of ways you can create security holes via your involvement in the game's external community. Instant messaging, websites, e-mail, and third party programs can all be used to gain access to your computer. There are ways to make it more difficult for a hacker to get into your computer, and any criminal is going to look for easy targets first, not difficult ones.