I'm just wondering, is this like a 2nd hard drive that I can put stuff in like I would with C:? Or is it another drive to put another OS on. I'm so confused.
It appears to be set up to take more "stuff" of whatever you want to store there. If you really wanted to, you could put another OS on the drive but do you really want to?
Yes. Some programs might give you trouble installing on anything but C but for those that will allow it, the answer is Yes. You can also store files like music or videos on D if you want.