Microsoft demonstrated its DirectX 12 API at last week's Windows 10 event. This new release promises big reductions in overhead and a significant increase in gaming performance and better power efficiency. The software giant also talked about DirectX 11.3, which is basically a partial DirectX 12 without some fewer features and no Mantle-like overhead reduction. It's speculated that DirectX 12 may be exclusively for Windows 10 whereas DX11.3 may arrive for earlier versions of Windows.
Full details on what to expect from DirectX 12 and how Microsoft is cooperating with Unity Tech and Epic Games to make it easier for game developers to take advantage of these new features can be read at DailyTech.
Currently Windows 8.1 is equipped with DirectX 11.2. Coming next will be not one, but two releases. The first, DirectX 12 (DX12), targets low-level/high-performance code. Think game engine development. In many ways it's similar to vendor-specific efforts like Advanced Micro Devices, Inc.'s (AMD) Mantle API. The goal of all of these efforts is often summarized as getting the developer "closer to the metal".
The second release will be DirectX 11.3 (DX11.3), which will feature more approachable APIs for independent developers. It will feature a paired down set of DX12-like features, but won't pack quite the raw firepower of DX12. Both APIs will reportedly operate side by side, giving developers flexibility along with a degree of commonality.