Yes, completely. It is not only free to use, it is "Open Source" and it is "Free Software".
You can find the source code on GitHub.
No. But you can download a VirtualBox Virtual Appliance, see :ref:`appliance-label` with a Linux inside, and run this virtual machine on your Windows desktop.
No, there are many commands but most users need only few of them, see next Question :)
See :ref:`usage-label`.