ReactOS is a free and open-sourced operating system based on the Windows architecture. It stands for React Operating System. The operating system a ground-up implementation of Microsoft Windows XP compatible OS. The ReactOS team aims to achieve complete binary compatibility with both applications and device drivers. The OS is licensed under the GNU General Public License, but some parts have different licenses like code from Wine project which is licensed under LGPL.
In major difference in ReactOs is that even though its open-source, it is not a wrapper built on Linux. Instead the ReactOS development team has built a operating system with windows architecture and plan to provide support for existing applications and drivers. So is it another OS apart from all these Windows,Linux,Unix,Mac...?? Well even though the ReactOS belives it's not "yet another OS".. I belive it is. As per the ReactOS team the major reason for developing this OS is that "some people, or especially companies, will -never- switch to Linux." -- Which I will never belive!
ReactOS claims that its compatible and visualy consistent with Windows.Thus,it removes many of the problems that people or companies face when switching to Linux: training everyone on the new OS, increased support costs, acquiring or writing new software, hardware incompatibilities, and the list grows.
So, is this new OS here to stay and replace what Linux has been fighting for (desktop replacement of windows)? You can download the 0.3.1 version of ReactOS and check out how good it is! Good luck!