ReactOS® is an effort to create a Free and Open Source replacement for the Microsoft Windows NT® family that is compatible with both applications and drivers. The NT® architecture has always been highly flexible and powerful and its continued dominance in the computer industry means it is one of the most supported family of operating systems in existence, with its latest iteration being Windows 8.
As these days operating systems are little more than gateways to applications that users want to run, an open source NT implementation would allow users to continue using familiar programs in a familiar environment. The project seeks to embrace the strengths of the NT family while avoiding many of the configuration decisions that made older versions of Windows vulnerable and maintaining a lightweight environment so that a computer's resources can be dedicated to what really matters to the user, running their applications.
ReactOS 0.3.16 is still under heavy development (alpha stage) and is not ready for everyday use.
You can help the ReactOS project by word-of-mouth advertising, donating money to the ReactOS Foundation or by spending time on the ReactOS project and contributing in one of various ways (submitting a patch, helping with website and OS translations, posting in the community forum, etc.).