I hope someone with more experience with these answers you. I rarely go there. No one looks at either my album OR my gallery! lol Seriously, though, I believe they're open to the public.
I think that you can make a specific album password protected so it wouldn't be available to all. There are a few options you can choose when you decide to make a new album in your gallery.
Basically, the gallery is like a bookshelf where the albums are the books in it.