Gradually, as Windows has become larger and better factored, Windows USER has cooperated with other components to provide this functionality:
Controls: Starting with Windows 95, new controls were placed into a separate common controls component. Starting with Windows XP, new implementations of the standard controls were also moved to that same component.
Windows Management experience: Starting with Windows Vista, drawing standard window management was moved from Windows USER to the Desktop Window Manager.
Standard Visuals: Starting with Windows XP, visual elements are drawn by the Theming component.
Menu bar: Starting in Windows 7, the Ribbon interface is a recommended replacement for the menu bar and context menus. It is provided by the UIRibbon component.
Developer experience
Windows USER implements abstractions that are used to build Windows applications. These include
HWND
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 for delivery of messages. Windows are managed as a tree, with the desktop window at the root of the hierarchy. Child windows are usually visually contained within their parents.
MSG
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.
Message Loop
By combining calls to Windows USER in the right way, a Windows application processes messages. The core Windows message loop is at the core of any Windows application. Developer functionality related to Windows USER is provided in the C header file winuser.h.
Implementation
In 16-bit versions of Windows, Windows USER was implemented as a file called user.exe. The file extension here was a misnomer, as Windows USER was in fact always a dynamic link library. In 32-bit versions of Windows, the 32-bit version of Windows USER is called user32.dll and is located in the System32 directory, while compatibility with 16-bit applications is provided by having a modified version of user.exe also present. In 64-bit versions of Windows, the 64-bit implementation of Windows USER is called user32.dll and is located in the System32 directory, while a modified 32-bit version is present in the SysWOW64 directory.
