Microsoft is aware of limited, targeted attacks that attempt to exploit a vulnerability in Internet Explorer 6, Internet Explorer 7, Internet Explorer 8, Internet Explorer 9, Internet Explorer 10, and Internet Explorer 11.
The vulnerability is a remote code execution vulnerability. The vulnerability exists in the way that Internet Explorer accesses an object in memory that has been deleted or has not been properly allocated. The vulnerability may corrupt memory in a way that could allow an attacker to execute arbitrary code in the context of the current user within Internet Explorer. An attacker could host a specially crafted website that is designed to exploit this vulnerability through Internet Explorer and then convince a user to view the website.
To put the matter very crudely, the bug potentially could allow a hacker to take over your entire computer. According to the security firm that discovered it, this vulnerability has been actively exploited. Most people are probably unaffected, but if you use IE you might want to reconsider your browser, at least until the patch comes out.
The Firefox family of browsers (whether made by Mozilla or not) are all unscathed, as is Chrome.