Usually they are people that wear really elaborate costumes (like custom made that look like the "real" thing) and then act like the characters they are playing. A little different than just costumes. They will act out their interactions with each other as the characters they are portraying. Basically the difference between two women dressed as vampires at a halloween getting into a fight vs. them play acting out a fight as if they were vampires (hissing, going for a bite vs. a punch, etc.)