Direct2D is an application programming interface (API) designed for the rendering of 2D graphics (Direct3D is used for 3D graphics rendering) for applications that required high performance graphics rendering. It is part of the Microsoft DirectX package of APIs for video and audio development. Originally designed for the Windows 7 and Windows Server 2008 operating systems, Direct2D has been ported to Microsoft's Windows Vista operating system too.
Other DirectX related glossary:
DirectX Media Objects