ReactOS is an open source operating system, based on Windows. It is not Unix based, it does not use the Unix kernel and architecture. Written completely from scratch, in C and C++, ReactOS seems to be the free alternative of Microsoft Windows.
It runs the Windows binaries and has native support for the Windows app. After years of development, ReactOS is still considered an alpha, even though it is stable.
The latest ReactOS, ReactIS 0.3.15 brings improved support for USB mass devices, keyboards and mice
After a year of development is finally available the new version of ReactOS 0.3.15 release that introduces several architectural enhancements developed an implementation to bring more compatible and compliant NT architecture.
If I made you curious, see the ReactOS official site. It can be downloaded as a live cd or as a virtual machine. See this link.