There are many definitions of cyber security. One US government agency simply states "Protecting...critical digital assets and the information they contain from sabotage or malicious use is called cyber security."
We define it as: "Protecting digital information, property and networks from theft, corruption and natural disaster while at the same time keeping them accessible and productive for intended users."
In general, cyber security aims to prevent three types of malicious activities:
Exploitation: Using digital systems for unintended and/or nefarious purposes (such as stealing money or intellectual property);
Disruption: Degrading cyber assets and/or preventing their use (such as vandalizing sites or shutting them down entirely); and
Destruction: Destroying or damaging physical assets (such as using malicious software to make machines damage themselves).