GDI+ is the successor to GDI, the graphics device interface included with earlier versions of Windows.
A graphics device interface, such as GDI+, allows application programmers to display information on a screen or printer without having to be concerned about the details of a particular display device. The application programmer makes calls to the provided functions and those functions in turn make the appropriate calls to specific device drivers. GDI+ insulates the application from the graphics hardware, and it is this insulation that allows developers to create device-independent applications.
Microsoft Windows GDI+ exposes a flat application programming interface (API) that consists of about 600 functions, which are implemented in Gdiplus.dll and declared in Gdiplusflat.h. Microsoft has wrapped these functions with a collection of about 40 C++ classes and the Microsoft Product Support Services will not provide support for code that calls the flat API directly.
The functions in the flat API return a status value that indicates whether the call has succeeded or not.
MSDN Documentation: GDI+