When Microsoft first introduced Windows Phone 7 platform, the apps were designed with Metro guidelines and were called Metro apps. Due to the trademark dispute with Metro UK company, Microsoft dropped Metro branding and called them as ‘Windows 8 style’ apps with Windows 8. After that, it became Windows Store apps. Later with Windows 8.1, Microsoft started calling them as Modern apps. To some extent, Microsoft also referred them as Windows Universal apps. Now, Microsoft has named them as just ‘Windows apps’. Classic apps that runs on desktop are called Windows desktop applications. You can see their definition of both above.