BuddyPress.org

Okay, I’m a little confused. I’m new to all of this, so I hope I don’t sound dumb. I read child theme article and I’m a little confused:

Is a child theme the same as a theme you can download and install? Or is it something different enitrely? Because I currently have a modded installed theme on the site currently that I’m using and want to use (it’s installed in wp-content/themes like a child theme would be …).

So is it possible for me to make some sort of override to the registration template? Because I just want the registration page to load without the theme/css/ any part/links of the site being shown — just the form.