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