You can download ReactOS in two different flavors. As a Live CD so you can test it without installing into your computer, or as a Boot CD so you can install ReactOS on your PC.
If you don't know what to do with the downloaded file...
Here is the source code package.
Each new commit creates its own ISO. This way you can test almost immediately any new fix included into ReactOS. These are released daily so they do not receive extensive human testing. However, they go through our testman. You can expect ReactOS to work better but there may also be regressions.