The "Critical" vulnerability affects Internet Explorer versions 7 through 11 on Windows 7, 8, 8.1, 10 and Vista. Windows Server 2008, 2012, 2012 R2 and the Windows Server Technical Preview are all effected, but Internet Explorer runs in a "Enhanced Security Configuration" that should mitigate the effects of this problem. The "out-of-band" patch was released outside Microsoft's typical Patch Tuesday release cycle and allows users and administrators to update their computers quickly.
Microsoft Edge, the new browser included in Windows 10, isn't affected by the vulnerability. However, Microsoft's new operating system also ships with a copy of Internet Explorer 11 installed, which is why it's getting updated.
The vulnerability is caused by an issue with Internet Explorer improperly accessing objects in memory. That could corrupt memory in a way that allows attackers to execute code with the current user's privileges. If that user has administrator capabilities, the attacker would be able to take complete control of a computer and do things like modify files and install programs. Users without administrator privileges may be slightly better off, though an attacker will still be able to do anything they're able to do.
Windows 10 received the patch as part of a cumulative update that also installs all of the prior patches Microsoft released for the system, the fourth that Microsoft has released since the launch of Windows 10 on July 29.