ReactOS® is a free, modern operating system based on the design of Windows® XP/2003. Written completely from scratch, it aims to follow the Windows® architecture designed by Microsoft from the hardware level right through to the application level. This is not a Linux based system, and shares none of the unix architecture.The main goal of the ReactOS project is to provide an operating system which is binary compatible with Windows. This will allow your Windows applications and drivers to run as they would on your Windows system. Additionally, the look and feel of the Windows operating system is used, 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.
Please bear in mind that ReactOS 0.3.8 is still in alpha stage, meaning it is not feature-complete and is not recommended for everyday use.
Aside from submitting bug reports, posting on the forum and sometimes editing the project wiki, I don't actively participate in ReactOS development myself. However, with the upcoming release of Windows Internals 5th Edition, with contributions from ReactOS developer Alex Ionescu, and the new free time I have now that my University course is nearing it's conclusion, this may change in the future.