When you think of network security, what comes to mind? Dishonest employees trying to misuse company information for their own gain? Competitors who would love to get their hands on your customer lists and sales information? Hackers who have nothing better to do with their time and energy than to disrupt your business processes? In reality, network security covers all of these areas - and many more. Physical storage, environmental security, virus protection, hacker avoidance, and implementing authorized access policies each play a role in ensuring that your computer system is protected and working properly when needed.
Like many areas of computing, security is also constantly changing. For every new security practice or tool on the market, there's always someone out there trying to find a new way to compromise your system. Just look at the rapid proliferation of viruses and worms on the Internet: even though there are over 100,000 known (and detectable) virus/worm variations in existence, an average of three new viruses/worms are discovered EACH DAY! Spyware (which can invade your privacy and steal your personal information without your knowledge) is running rampant - so much so that even Congress has tried to address it. And this doesn't even count the numerous application and operating system loopholes that major software manufacturers are constantly finding in their products. Needless to say, someone is always seeking (and finding) new ways around the safeguards that you may already have in place.
In short, network security isn't a product or feature that you can buy and forget about - it's an ongoing effort that requires you to stay diligent in the battle to stay one step ahead of the constantly evolving threats to your system that exist in today's interconnected world.