DirectX is a set of multimedia APIs from Microsoft aimed primarily at game developers. Popular APIs within the DirectX collection include Direct3D, XInput, and XAudio.
Microsoft DirectX is a collection of APIs for handling tasks related to multimedia, especially 3D graphics and game programming and video, on Microsoft platforms. Originally, the names of these APIs all began with Direct, such as Direct3D, DirectDraw, DirectMusic, DirectPlay, DirectSound, and so forth. The name DirectX was coined as shorthand term for all of these APIs (the X standing in for the particular API names) and soon became the name of the collection.
More information at Wikipedia article of DirectX
References & Tutorials:
- Rastertek Tutorials (DirectX 10 and 11 Tutorials)
- Direct3D 11 Graphics MSDN
- C# Ported Version of Rastertek Tutorials
- DirectX 11 Lessons
- Microsoft DirectX Forum
How Do I Get Started with DirectX?
The DirectX SDK can be found at the DirectX Developer Center on MSDN.