Windows functions, like C run-time functions, are defined in libraries. The Windows libraries, unlike the C run-time libraries, are special dynamic-link libraries (DLLs) that the system links with your application when it loads your application. Dynamic-link libraries are an important feature of Windows because they minimize the amount of code each application requires.
Windows consists of the following three main libraries:
| Library | Description |
| User | Provides window management. This library manages the overall Windows graphical environment, as well as an application's windows. |
| Kernel | Provides system services, such as multitasking, memory management, and resource management. |
| GDI | Provides the graphics device interface (GDI). |