Yesterday, Microsoft took the unusual step of releasing two emergency updates, one for a Critical Vulnerability in Internet Explorer and another for Visual Studio. It is rare for Microsoft to deviate from it's monthly patch schedule, which should give you an idea of how important this particular update is. Normally, patches are issued once a month (on the second Tuesday).
You may have to install one or both of the security updates depending on what Microsoft software you have installed. As always, the sooner you apply the updates, the faster your PC will be protected from these vulnerabilities.