DirectX is a Microsoft's API which contain features to aid in game development UI design. Questions asked with this tag should focus on conceptual aspects regarding the DirectX API.
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
Questions asked about DirectX should focus on the conceptual aspects of using the DirectX APIs within an application.