All currently supported versions of Windows, including Windows 10, are getting a out-of-schedule security update today. The patch is designed specifically to fix an issue that was discovered in all versions of Internet Explorer.
Normally, all Windows security patches are issued on the second Tuesday of each month, also known as "Patch Tuesday". However, Microsoft felt the need to release this patch outside the normal time frame. It is labeled as a "Critical" update for all versions of Windows, and "Moderate" for Windows servers. Microsoft says:
"This security update resolves a vulnerability in Internet Explorer. The vulnerability could allow remote code execution if a user views a specially crafted webpage using Internet Explorer. An attacker who successfully exploited this vulnerability could gain the same user rights as the current user. Customers whose accounts are configured to have fewer user rights on the system could be less impacted than those who operate with administrative user rights."
The patch does not affect Windows 10's new web browser, Microsoft Edge.