ReactOS is an advanced free open source operating system providing a ground-up implementation of a Microsoft Windows XP compatible operating system. ReactOS aims to achieve complete binary compatibility with both applications and device drivers meant for NT and XP operating systems, by using a similar architecture and providing a complete and equivalent public interface.
ReactOS is the most complete working model of a Windows like operating system available. Consequently, working programmers will learn a great deal by studying ReactOS source code and even participating in ReactOS development.
ReactOS has and will continue to incorporating features from newer versions and sometimes even define the state of the art in operating system technology.

Actually, the ReactOS project reimplements a state-of-the-art and open NT alike operating system based on the NT architecture-design. It comes of course with a WIN32 subsystem, NT driver compatibility and a handful of useful applications and tools.
ReactOS combines the power and strengths of the NT kernel - which is well known for its extensibility, portability, reliability, robustness, performance and compatibility with Win32 compatibility.

Recent NT based operating system from Redmond, especially XP, got bad reputation for its weak default security-settings; mainly, to simplify the transition from Win9x for both users and legacy applications. This decision alone invalidated many of the security features in NT. ReactOS will incorporate proper default security settings.
ReactOS has been designed for high security; it doesn't share common security flaws with other operating system.

In short, ReactOS is designed to be powerful and lightweight. You can think of the term "lightweight" in the good old fashion of Win95, a consistent user interface and small bundle of very common and useful tools. In contrast, ReactOS offers a lot more, an up-to-date experience as well as built from scratch on a rock solid NT core.

ReactOS is free software, the source code of the whole system is available for free and it is licensed under the GNU GPL license.

In fact, ReactOS has been written from scratch since 1996, a rock solid NT re-implementation, and therefore a reliable and robust operating system for tasks ranging from embedded micro computer to personal computer, workstations to server cluster, mainframes and super computers.
ReactOS incorporates many design decisions from other operating system families like UNIX, VMS, OS/2 and of course NT and is meant as 'the' new platform that serves all.

The ReactOS operating system design is able to provide portability across families of processors, such as Intel x86 and even provide portability across different processor architectures, such as CISC and RISC.
There is only one single OS core, the kernel; porting ReactOS to other architectures involves 'only' the hardware abstraction layer, the lowest part that talks directly with the platform hardware.

ReactOS is flexible and extensible by design. ReactOS is probably one of the most versatile operating system platform, especially thanks to its NT kernel and the open source nature.
ReactOS can be extended with the help of so called "subsystems" to provide support for legacy applications from other platforms. For example, a POSIX subsystem would provide compatibility layer with various flavors of UNIX applications.

ReactOS is not an object-oriented system in the strictest sense of the term, but it does use objects to represent internal system resources. Software objects are a combination of computer instructions and data that model the behavior of things, real or imagined, in the world.

The license of this software is Freeware, you can free download and free use this system utility software.