The purpose of magic circles(black magic, maybe?)?

I have no intention of getting involved with black magic, but I'm writing a manga(graphic novel) about a girl who has a book full of spells and magic, but I don't know what they are called or their purpose. I saw one anime(japanese cartoon) called Umineko no Naku Koro ni(when the sea gulls cry) and it involved magic circles. I think one was called the seventh pentacale of mars, or something. Anyway, they resembled alchemic circles. I can post a a link that will allow you to see what I'm talking about if you need me to. Thank you very much please explain what these circles me. Also, what is the magic book in black magic called? (A grimoire?) Please and thank you!