Windows is running in a partition of your Macbook basically taking up some of the disk space. You can either boot to MacOS or Windows at your leisure by holding the command key down when you are powering on and selecting the operating system. It is a Windows box at that point just running on Apple hardware (much in the same way as it runs on HP, Dell, Or Asus hardware). The main difference is the support of the drivers and that is part of what boot camp is providing - drivers for the video, mouse, camera, EFI chip, etc.

If, for some reason, your Windows is corrupted to the point it cannot run, you can download the ISO and reuse your product key. with no problems. If for some reason there is a problem, calling Microsoft's activation line clears it up in about a half hour (but I have not had a problem with this yet).