ReactOS is a free and open-source operating system based on the Windows NT architecture, providing support for existing applications and drivers, and an alternative to the current dominant consumer operating system. It would be perhaps important to start by saying what ReactOS isn't. It is not another wrapper built on Linux, like WINE. It does not attempt or plan to compete with WINE; in fact, the user-mode part of ReactOS is almost entirely WINE-based and our two teams have cooperated closely in the past. ReactOS is also not "yet another OS". It does not attempt to be a third player, like SkyOS or any other alternative OS out there. People are not meant to uninstall Linux and use ReactOS instead; ReactOS is a replacement for Windows users who want a Windows replacement that behaves just like Windows.
While ReactOS's source availability and current usage is mostly intended for programmers to expand and improve on, you don't need to be a rocket scientist to use it, and in its final state will be a consumer friendly OS. If you've used Windows before, you'll find yourself in a familiar environment with ReactOS. The learning curve, if any, should be minimal, since ReactOS duplicates many of the Windows graphical environment applets, control panels and dialogs (Windows 2000/XP/2003).
Focus at present is on developing ReactOS to a stage where it is capable of running most Windows drivers and applications "out of the box." This does not mean that ReactOS will stop there, however; as new features are added to Windows, our developers be striving to incorporate such new features in future versions of ReactOS. Once the compatibility level has been reached, we may even choose to improve ReactOS beyond its Windows roots. These changes would be external from the main OS, and totally compatible with Windows, but they would add an extra advantage. For example, one of our goals is to have built-in POSIX compatibility, much like Windows Services for Unix. It could also be as simple as natively supporting multiple desktops however, instead of requiring a powertoy.
The Live CD lets you try ReactOS without installing it on your hard drive.