Direct3D is the 3D graphics API from Microsoft. It is part of the DirectX package and was introduced with DirectX version 3. The version 3 API was really horrible. The programmer had to mess with execute buffers and lots of other ugly low level details. Microsoft cleaned up the API some and included concepts from OpenGL in version 5 of DirectX. This is much better than the version 3 API, but is still tied to Windows and inferior to OpenGL.