Previously when new versions of the Windows operating system were released, they were based on the same codebase, and were basically treated as customizations of the OS. Developers could create a Win32 application that would still run on the various versions of Windows that were still in use. When Windows 8 is released, and WinRT and Metro style apps are being created, this won't be the case.
With the upcoming release of Windows 8, developers have the opportunity to create Metro and WinRT applications, but there is still an issue to be dealt with: fragmentation. While Windows 8 offers a unified OS for both Windows Phone and computer users, previous versions of Windows will be in use for many years, and that's a problem for developers.