How do you define security? To some people, firewalls immediately comes to mind. To others, process and procedures are the first things they think of. Other people think about guards and guns.
I like to think of security as risk management. A vulnerability exists. This is a possible flaw in your system. A threat exists. This is something to take advantage of the vulnerability. This is your threat vulnerability pair. When you put these together you get your risk. To protect yourself you need to mitigate and manage this risk.