As performance demands grew, and as video cards became more and more sophisticated, standards started to emerge. Two of the biggest early standards were (and still are, although they've evolved) OpenGL and DirectX. Both of these were implemented in the video card drivers, but were exposed by Windows. DirectX came from Microsoft and is only officially supported on Windows, and OpenGL came from SGI and is an open specification. (DirectX also includes a bunch of other stuff that isn't video card related, but I'm ignoring that for the purposes of this answer)