There is an inter dependency between these three attributes. When security goes up, usability and functionality come down. Any organization should balance between these three qualities to arrive at a balanced information system.
Some important terms to consider in hacking are
Threat: Anything that has potential to cause harm. There are various threats available to system threats, Network threats, application threats, cloud threats, malicious files threats etc.
Vulnerability: A weakness or a flaw in the system which an attacker may find and exploit. An updated OS, Default Passwords, Unencrypted protocols are all good examples of vulnerabilities.
Attack: Method followed by a hacker/Individual to break into the system. Denial of service attack, Misconfiguration attacks, Operating system attacks, Virus, and Worms are all example of Attacks.
Attack vectors: Path or means by an attacker gains access to an information system to perform malicious activities.