ReactOS is a free, open-source reimplementation of the Windows NT architecture. In other words, it’s an attempt to reimplement Windows as an open-source operating system that’s compatible with all Windows applications and drivers. ReactOS shares some code with the Wine project, which allows you to run Windows applications on Linux or Mac OS X. It’s not based on Linux — it wants to be an open-source operating system built just like Windows NT. (Modern consumer versions of Windows have been built on Windows NT since Windows XP.)
This operating system is considered alpha. Its current goal is to become compatible with Windows Server 2003, so it has a long way to go.