The bold concept Windows 10 introduced was Universal Windows Platform (UWP) - "Write once, Deploy anywhere" truly and literally. To make it more simple and clear, if you create Universal Windows application, then you can run the same app in variety of devices ranging from Desktop, Tablet, Mobile and upto your holographic computer (HoloLens) without recompiling your code. Initial requirement for UWP app development was Windows 10 and Visual Studio 2015.
Things have changed now! With the recent announcement at //build/ 2015, you can develop UWP app in Windows 8.1 and Windows 7 operating system using VS2015. But, please note the experience may defer. For example, if you are developing UWP app in Win 8.1/7, then you cannot deploy the app to your Local Machine or to any simulators. Instead, you need a real device for development and testing. The exception is, Mobile emulators will be there. Also, XAML Designer support will not be there, but Intellisense is. For developing UWP in Windows 7, you need to wait till VS2015 hits RTM.
Here is a screenshot of UWP OS support which was share during //build/ 2015 event.
This is a welcome change. There is always a large community who cannot immediately migrate to Windows 10 exclusively for UWP app development. This move will definitely help them and those who are hesitant to update to developer preview of Windows 10.