ROCKLAND — What exactly is a novella? How does it differ from a short story or a novel, and why has the form produced some of literature's most enduring works? Those questions will be at the heart of “What Is a Novella?: Writers on Fiction's Most Elusive Form,” a panel discussion hosted by Millay House Rockland and The Arctic Tern Bookshop, on Thursday, July 30, from 6:30 to 8 p.m. The event will be held both virtually and in-person at The Arcti…