Help Viewer is a WebKit based HTML viewer for macOS aimed at displaying help files and other documentation. It is found in /System/Library/CoreServices/Help Viewer.app. The default file extension is ".help". Help index files are generated with Help Indexer. macOS applications typically use Help Viewer to display their help content, rather than a custom system.
Help Viewer's implementation in Mac OS X 10.5 (Leopard) found its way to Rob Griffiths' list of Leopard criticisms, because Apple changed the software from a standalone application with a standard window interface to one with a floating window that always appears in front of all other application windows, obscuring the interface for which one is seeking help (see image below).
Although one can close or minimize the Help Viewer window, it is difficult to consult the Help Viewer while simultaneously working with the application, short of changing the size of windows so both fit on the screen. The Help Viewer window also does not work with the Exposé window management feature (Mission Control in OS X 10.7 or later). There is a workaround using the defaults command accessible in the Terminal.