Internet Explorer 11 on Windows 8.1 and Windows 7 comes with a completely redesigned and enhanced suite of in-browser developer tools that help developers build, diagnose, and optimize modern Web sites and apps across multiple devices. The new tools, which we call F12 for short, enable Web developers to work quickly and efficiently.Source : IEBlog
The Visual Studio and IE teams have worked together to build F12 with a core principle of helping you get from problem to solution quickly with actionable data. The new F12 enables you to deliver fast and fluid Web experiences with tools for diagnosing and fixing performance issues and tools that give you deeper insight into how IE is laying out and rendering your app. F12 supports the fast, iterative workflow used by modern Web developers.
Jean-Pierre Paradis's Blog
Internet Explorer
Windows 8
Debugging and Tuning Web Sites and Apps with F12 Developer Tools in IE11