You can see Windows 9's Start menu in action in this 2-minute leaked video. The new Start menu mixes modern (Metro) design elements, such as live tiles, with the standard Start menu layout available in Windows 7. Windows 9's Start menu can be expanded to the entire screen by adding more apps and live tiles, thus becoming more of a Start screen that can be easily accessed by clicking on the Start button, which was brought back in Windows 8.1.
Microsoft may even remove the fullscreen Start screen altogether for desktop users in favor of a Start menu that expands. PCs with a mouse and keyboard might come with the Start menu enabled by default, while tablets could instead boot directly to the desktop and stick to the Modern UI.
Source: WinFuture (German)