In the old days when you bought Antivirus software, that's pretty much all you got - a software program that help protected you from known computer viruses.
But with the advent of adware, spyware, and phishing scams, you now need a program that can handle these threats as well as protect you from computer viruses. This where "internet security" software comes in.
Unlike Antivirus software, internet security software is designed to help protect you from all threats (including computer viruses). Most of the top Antivirus companies offer you the choice of buying Antivirus or Internet Security software ... and given the choice, you're better off buying internet security software.
While Norton Internet Security is still my preferred choice, there are others available for those aren't fond of Norton. Below are links to some of the other alternatives that folks are using.