Security means different things to different people.
To some it could be keeping your child from getting access to your account when you leave it for a few minutes to make some coffee.
To others it is stopping sophisticated gangs of criminals from breaking into their systems.
Some cases are simply dealt with (installing xlockmore for instance), some require a whole shift in thinking so that everything you do has security in mind.
A common maxim for computer security is Security in depth which means building layer upon layer of security measures. It means that you are not only relying on one layer of security but many different measures which would all have to be circumvented in order to subvert the machine. "Weakest link" is also often a very important concept. With good layers, a failing link may not mean a complete disaster. Another common phrase is Security is a process, not a solution
"Weakest link" is also often a very important concept. With good layers, a failing link may not mean a complete disaster.
Another common phrase is Security is a process, not a solution. The label "security" seems to attract many kinds of commercial interests. Many if not most of those are selling "solutions" which, especially when it comes to security, mean very little on their own.
'Is more waffle needed here?'
For Debian systems there is a SecurityChecklist here and the "Securing Debian HOWTO", http://www.debian.org/doc/manuals/securing-debian-howto/ which may help you to get some ideas of how to start securing a machine.