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.
Information security types tend to think of protecting three different items for an information system. They are confidentiality, integrity, and availability. Confidentiality deals with who can and who cannot have access to the system. Integrity deals with whether or not the system has been altered in a way not intended. Availability deals with whether or not the system is accessible when it is supposed to be accessible.
These three items are usually weighted. The weights tend to be Low, Moderate, or High. Different groups and systems are weighted differently. For instance, government systems tend to weigh confidentiality and integrity higher than availability. Commercial companies tend to weigh integrity and availability higher than confidentiality. An example would be a public web server being used as a marketing device. This system may be rated as CIA:LMM.
After a system has been rated, vulnerabilities to CIA should be identified. For instance, say your public web server was located in New Orleans, LA. A vulnerability to availability may be that it is a single instance server. A threat to this availability would be a hurricane. The likelihood of a hurricane happening is high. The likelihood that a hurricane will affect the web server is moderate. Putting the two together, you have a real risk to the system.
A method to mitigate the risk of the system is to have plan in place to deal with losing the web server. You should take backups of the existing system. You may have a backup site somewhere else that is geographically separated. Utilizing those two items, you may be able to get your web server up and running within 48 hours of a catastrophic failure (Katrina). In this case, you have successfully mitigated and managed your risk to the availability of your system.