Windows USER is a component of the Microsoft Windows operating system that provides core functionality for building simple user interfaces. The component has existed in all versions of Windows, and includes functionality for window management, message passing, input processing and standard controls.

Windows USER provides a large part of the core user experience for Microsoft Windows. Historically, it was responsible for:

Gradually, as Windows has become larger and better factored, Windows USER has cooperated with other components to provide this functionality:

Windows USER implements abstractions that are used to build Windows applications. These include

The HWND is the basic unit of visual display and input in the Windows developer platform. All visual elements either have their own HWNDs or live as part of a larger HWND. Many functions in Windows USER manipulate properties on HWNDs such as size, position and title. Each HWND also has a communication channel (WNDPROC - Window Procedure) for delivery of messages.

Windows are managed as a tree, with the desktop window at the root of the hierarchy. Child windows are usually (but not always) visually contained within their parents.

Applications and the operating system communicate with HWNDs using messages. A window message is the combination of a target HWND, a message code, and other details such as mouse position.

By combining calls to Windows USER in the right way, a Windows application processes messages. The core Windows message loop (including calls to GetMessage, TranslateMessage and DispatchMessage) is at the core of any Windows application.