Hacking refers to exploiting security flaws to obtain personal information without permission. There are different types of hackers with varying intentions - some hack just for fun or to learn, while others seek to steal information or damage systems. A hacker's skills may include intruding networks, crashing systems, and using exploitation programs. To protect against hacking, system administrators should patch security holes, encrypt data, remove unnecessary programs/users, set up firewalls and intrusion detection, and backup systems regularly.