The idea is fairly simple: developers can use a Microsoft-provided software development kit—Java and C++ for Android and Objective C for iOS—to port their existing apps and games directly to Windows universal apps. In practice, it's obviously not as simple as snapping your fingers and having a complete port, but Microsoft plans on creating replacement APIs for apps to leverage if they rely on existing APIs from Google or other sources.
"We want to enable developers to leverage their current code and current skills to start building those Windows applications in the Store," Microsoft’s Terry Myerson told The Verge.
Microsoft is also interested in enabling websites and Windows desktop apps to migrate to Windows universal apps. One plan is a way for websites to run inside Windows universal apps. This would allow them to use system services like notifications and in-app purchases, as well as have an app presence without having to build a native app. The biggest issue, though, will be ensuring desktop programs still perform optimally in app form.
"We want to structure the platform so it’s not an all or nothing," Myerson said to The Verge. "If you use everything together it’s beautiful, but that’s not required to get started."