Phishing Filters/Web Filters
An increasingly important type of computer security, phishing and web filters are a form of active protection which specifically protects against web threats. Phishing scams and web scripting attacks are both attacks which are difficult for traditional antivirus methods to deal with, so having this extra layer of protection is important.
In both cases, protection is provided by checking the websites you visit for suspicious activity and by comparing them to a database of know dangerous websites. For example, you might be sent an email to what appears to be your bank's online login page. It would be very difficult for you to tell that this was a fake simply by looking at the page. A phishing filter, however, can look at the code of the website, the website's security credentials, and the website's domain name. If the website you are visiting is a fraud, the filter will let you know.
Web filters provide similar protection against threats which are executed through online web scripts which use security vulnerabilities in web browsers in order to gain access to your PC. If the web filter sees that a website you are visiting is a known threat or appears to have malicious code, it will stop the website from loading and inform you.
Most of today's antivirus programs roll this type of protection into the antivirus suite, but don't assume that to be the case. Some companies will sell an antivirus-only version of their software as well as a more comprehensive security suite which includes phishing and web filter protection. Be sure that you know what you are buying.