Microsoft's graphics application programming interface (or API), Direct X, has long been a staple of PC gaming. Today at GDC they've announced DirectX 12, and Nvidia (the GPU manufacturer) is on board.
DirectX has long been a household name to PC gamers. OpenGL was the biggest competing API, but it's become relatively extinct (as far as PC gaming is concerned); a good synopsis of the rivalry between the two is an old article from Tom's Hardware six years ago. It's been arguably that long since OpenGL has been relevant to PC gaming.
In comes Mantle, AMD's newly developed API. So far it's been released for a couple of games (the Thief remake, and Battlefield 4). Mantle is supposed to take advantage of hardware a bit more efficiently so that PCs can outpace consoles that are traditionally weaker. The Battlefield 4 results are great if not astonishing. As a result, Star Citizen is set to take advantage of the API along with future DICE titles. Plus, unlike DirectX, Mantle isn't hamstringed when not running on Windows; this is the biggest factor in announcing DirectX 12 now.
Traditionally, Microsoft has encouraged gamers to upgrade their PCs for the latest version of DirectX. DirectX 11.1 and 11.2 were only available on Windows 8. The API also promises to match Mantle's claim: that it will better take advantage of hardware specifications for console (i.e. XBox One) as well as PC. Again, like Mantle, it promises to make full use of multi-core processors.
We've seen what Mantle can do so far. The debut of DirectX 12 is the return of glorious competition, and in the end only us consumers will win.