The Microsoft Security Bulletin Severity Rating System is a simple four-level severity rating system applied to every Microsoft Security Bulletin. It allows you to quickly and easily assess the potential risk of an identified security vulnerability.

There is a different impact for different vulnerabilities. However, because most users do not understand how critical some updates are, and instead of you deciding for yourself which updates you should apply immediately and which you can probably ignore, Microsoft has developed the Security Bulletin Severity Rating System to rate them for you.

As I said, there are four different ratings in this system. They are all listed below with explanations as Microsoft defines them. These are in descending order of which are most important to apply:

Critical: A vulnerability, if exploited, would allow the propagation of an Internet worm without user action, and possibly without prompting. Microsoft recommends applying Critical updates as soon as possible.