The ReactOS kernel is a re-implementation of Windows' ntoskrnl.exe, for Windows Server 2003. It has been combined with a port of the WINE project to provide a userland for the operating system, which is mostly identical to the standard userland available on Windows. As a result, ReactOS is able to run any programs which WINE can run, and WINE has access to what the internals of Windows probably look like, advancing both projects to provide support for Windows programs on UNIX-like systems, such as Microsoft's Xenix, GNU\Linux, FreeBSD, and other operating systems. ReactOS also has limited support for DirectX 9, allowing some games available for Windows XP to run normally under ReactOS without modification.
ReactOS is available for x86 processors, x86-64 processors, ARM, and PowerPC.
ReactOS was first released in 1998 as version 0.07. It was initially a clone of Windows 95, called "FreeWin95". It's name comes from the reaction the developers had to Windows being closed source. It's most recent release is: 0.4.2. It is currently in its alpha stage, and not recommended for use other than evaluation.
This article is a stub, an article too short to provide more than rudimentary information about a subject. You can help the Microsoft Wiki by expanding it.