Wikipedia
ReactOS is an open-source operating system for x86/ x64 PCs intended to be binary-compatible with computer programs and device drivers made for Windows Server 2003.
Development started in 1996, as a Windows 95 clone project, which was in 1998 continued as ReactOS with the incremental addition of features of later Windows versions. ReactOS has been noted as a potential open-source drop-in replacement for Windows and for its information on undocumented Windows APIs. As stated on the official website, "The main goal of the ReactOS project is to provide an operating system which is binary compatible with Windows ... such that people accustomed to the familiar user interface of Windows would find using ReactOS straightforward. The ultimate goal of ReactOS is to allow you to remove Windows and install ReactOS without the end user noticing the change." , ReactOS is considered alpha software, feature-incomplete but with many Windows applications already working (e.g. Adobe Reader 6.0, OpenOffice etc), and therefore recommended by the developers only for evaluation and testing purposes.
ReactOS is primarily written in C, with some elements, such as ReactOS File Explorer, written in C++. The project partially implements Windows API functionality and has been ported to the ARM and AMD64 processor architectures. ReactOS, as part of the FOSS ecosystem, re-uses and collaborates with many other FOSS projects, most notably the Wine project which develops a Windows compatibility layer for Unix-like operating systems.