DirectX
From Tech Artists Wiki
Microsoft DirectX is a collection of application programming interfaces (APIs) for handling tasks related to multimedia, especially 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. DirectX, then, was the generic term for all of these APIs and became the name of the collection. After the introduction of the Xbox, Microsoft has also released multiplatform game development APIs such as XInput, which are designed to supplement or replace individual DirectX components.
Though DirectX and Direct3D are technically two different things, they are usually used interchangeably (Direct3D is part of DirectX).
Generally, technical artists don't need to know DirectX. Even if one learns C# or C++, DirectX is only really necessary with tools and plugins requiring new rendering code (and not just shaders). DirectX knowledge is not at all required to write shaders, though the concepts are important.
For other general info, and version history, visit Direct3D and DirectX at Wikipedia.
