In the fast-changing world of data communications, inexpensive Internet connection, and rapid software development, security is becoming more of a serious issue. Security is now a fundamental necessity because global networks are inherently insecure. When data from your system passes from point X to point Y on the Internet, for example, it may pass through numerous other points along the way, giving other users the chance to intercept, read and even change it. Even other users on your system may maliciously change your data into something you did not intend. Unauthorised access to a system may be obtained by intruders, also known as "hackers" or "crackers", who then use advanced technical knowledge to take over your 'cyber' identity, steal data from you, or even deny you access to your own system!
Do bear in mind that no computer system will ever be completely secure. All you can do is make it much more difficult for a cracker to compromise your system. For the average home Linux user, not much is required to keep the casual cracker at bay. But for high-profile Linux users (banks; government offices, hospitals, telecommunications companies, etc), much more systemic effort is needed.
Another factor to consider is that the more secure your make your system, the more intrusive your security becomes. You need to make a decision as to where in this hierarchy your system will still be usable, and yet remain secure for your purposes. For instance, you could set up a "hosts allow" file which will list all the IP addresses and 'identities' of all computers that have permission to access your system. Conversely, Linux allows you to create a "hosts deny" file which will refuse connections from any IP address listed within it. You could also setup your Linux system with no connectivity to the Internet, but this will place obvious limitations as to its use.
Larger commercial sites must establish a security policy stating how much security is required for the OS installation, and what auditing needs to be in place to check it.
Be aware of your system. Check Linux system log fles such as /var/log/messages and keep a watchful eye on processes using the Linux "top" command from time to time, especially when system loads seem to be unusually high, indicated by a sluggish machine.
Maintain your system by ensuring that you have installed the latest versions of software and have upgraded according to security alerts. Just doing this will help make your system markedly more secure.
Finally, beware of third-party PHP scripts like forum and blog software. These can be a goldmine for hackers looking to compromise your Linux system.