Private
I don't actually think I've been to a ball before, either. But I figure it's a high class party. You need a suit. And she'll need a dress. You can hit up stores in town and ask for formal clothing and the clerks can help you. Some people also offered to help newbies shop for clothes.
There isn't a ton else. You show up. There's dancing. You eat. Mostly it's a chance to mingle with people in the community and look nice.