Old Norse and Old Icelandic

Hi everyone!

I am currently doing an essay on Old Icelandic or Old Norse, and I was wondering : what is the difference between the two of them?
I read that Old Icelandic is a dialect from Old Norse, and that the majority of the literature was written in Old Icelandic (in Introduction to Old Norse, by Gordon).

Is the Prose Edda, by Snorri Sturluson, written in Old Icelandic or Old Norse? Are these two languages very similar?

I am confused, because it sometimes seems that authors uses Old Norse and Old Icelandic as synonyms...

Thank you for your help! (And sorry if my english is bad, it is not my first language!)

Isabelle