gdiplus.dll is a graphical library introduced in ~2001 by Microsoft to make two-dimensional drawing easier. It runs on top of gdi32, and besides the changes in the programming model it also adds new features such as gradient fills, anti-aliasing, more extensive image handling, etc.
Microsoft's GDI+ uses mostly gdi32 blt operations to copy its rendering results onto the destination device context whereas Wine's GDI+ (written summer 2007 by Evan Stade) passes calls through to their gdi32 equivalents. The latter approach has better performance and shorter development time, but sacrifices control over finer details.
Jose Roca's GDI+ documentation is an invaluable resource and much easier to use than MSDN.
The Mono project's GDI+ uses Cairo as a backend. Looking at their code can sometimes be better documentation than anything else on the web, although the details (like parameter checking) don't always precisely conform to Windows.