If you've been following Windows releases closely over the past decade, the news that DirectX 12 will be included with the final version of Windows 10 won't come as a massive surprise. Microsoft recently confirmed this news in a blog post, although they weren't particularly forthcoming with details.
One thing that still remains up in the air is which previous versions of Windows, if any, will support DirectX 12. When DirectX 11 launched with Windows 7, it was shortly released on Windows Vista as well; however with DirectX 11.1, Microsoft only released some features of the new graphics API some weeks after Windows 8 was made available.
With Microsoft looking to shift people away from using Windows 7, it's doubtful that DirectX 12 will be backported to the most popular version of the operating system. However, it typically takes the gaming industry several years before we see broad adoption of a new Direct3D API, so you'll have plenty of time to prepare yourself for a major OS upgrade.
For developers in Microsoft's DirectX Early Access Program, the company has announced that you'll now be able to preview a branch of Unreal Engine 4.4 that utilizes DirectX 12. Details of the Early Access Program are kept under wraps, which means we won't know what a difference DirectX 12 makes until closer to its launch.
Microsoft is set to launch Windows 10 in mid-2015, although you can already download a Technical Preview sans DirectX 12. If you haven't been keeping up with the news, DirectX 12's main new feature is lower-level abstraction for better performance, similar to what AMD provides with Mantle. Other features of the API include conservative rasterization, draw bundles, compressed resources, additional blend modes, and more.