Our is the possession of more than one person, including the speaker. "My husband and I bought our new house today" "Our daughter is not doing well in school" If it is yours and only yours, you would say "my". If it was someone else who owned it, you would say "his" "hers" or "theirs"

Our is the way to say that an object belongs to us. So "it is our book" means the book belongs to us. Ours also means that it belongs to us, but does not require the object of the possession. Sot "it is ours" means that "it" belongs to us. In this case the "it" object will be clear from something said earlier or perhaps if you are holding it in your hand or pointing to it.