The Graphics Device Interface (GDI, sometimes called Graphical Device Interface) is one of the three core components or "subsystems", together with the kernel and the Windows API for the user interface (GDI window manager) of Microsoft Windows.

GDI is a Microsoft Windows interface for representing graphical objects and transmitting them to output devices such as monitors and printers.

GDI is responsible for tasks such as drawing lines and curves, rendering fonts and handling palettes. It is not directly responsible for drawing windows, menus, etc.; that task is reserved for the user subsystem, which resides in user32.dll and is built atop GDI. GDI is similar to Apple's classic QuickDraw.

Perhaps the most significant capability of GDI over more direct methods of accessing the hardware is its scaling capabilities, and abstraction of target devices. Using GDI, it is very easy to draw on multiple devices, such as a screen and a printer, and expect proper reproduction in each case. This capability is at the centre of all WYSIWYG applications for Microsoft Windows.

Simple games which do not require fast graphics rendering use GDI. However, GDI cannot animate properly (no notion of synchronizing with the framebuffer) and lacks rasterization for 3D. Modern games use DirectX or OpenGL, which give programmers access to more hardware capabilities.

Source: Wikipedia