The features provided by the Windows API can be categorized as seven classes

1. Base Services (base service), which provides access to the underlying resources available to Windows systems. Examples include: file system, external device, process, thread, and access to the registry (Windows registry) and error-handling mechanisms (err handling). These feature interfaces are located in the Kernel.exe, Krnl286.exe, or krnl386.exe system documentation under 16-bit Windows, and in the Kernel32.dll and Advapi32.dll under 32-bit Windows.
2. Graphics Device Interface (GDI), which provides the ability to output graphics content to displays, printers, and other external output devices. It is located in Gdi.exe under 16-bit Windows, and Gdi32.dll under 32-bit Windows.
3. Graphical user interface (GUI), which provides the ability to create and manage screens and most basic controls, such as buttons and scroll bars. Receive mouse and keyboard input, as well as other GUI-related features. These call interfaces are located in User.exe under 16-bit Windows, and user32.dll under 32-bit Windows. From the Windows XP version, the calling interface of the basic control and the common dialog box control (Common control Library) is placed in Comctl32.dll.
4. Common Dialog link library (Common Dialog Box Library) provides standard dialog boxes for applications such as open/Save document dialogs, color dialogs, font dialogs, and so on. This library is located in Commdlg.dll under 16-bit Windows, and in 32-bit Windows under Comdlg32.dll. It is categorized under the user Interface API.
5. Universal Control link Library (Common Control Library) provides an interface for applications to access some of the advanced controls provided by the operating system. such as: status bar, progress bar (progress bars), Toolbars (toolbar), and Labels (tab). This link library is located in Commctrl.dll under 16-bit Windows, and in 32-bit Windows under Comctl32.dll: It is categorized under the user Interface API.
The 6.Windows shell (Windows shell), as part of the Windows API, not only allows applications to access the features provided by the Windows shell, but also improves and enhances them. It is located in Shell.dll under 16-bit Windows, and in Shell32.dll under 32-bit Windows (Windows 95 is in Shlwapi.dll). It is categorized under the user Interface API.
7. Network services, which provides interfaces for accessing the various network functions provided by the operating system. It includes NetBIOS, Winsock, NetDDE, RPC, and so on.

The features provided by the Windows API can be categorized as seven classes