A troll is a mythical beast who is known for living under the ground. This term is sometimes synonolous with dwarf, dark elf or kobold. The fairy tale version where the troll lives under a bridge and demanding payment of a toll from travelling goats comes from a 19th century collection of folk tales.
Trolls in the SCA
This term is also given to the person who sits and receives money from those attending an SCA event (thus also the term "troll booth"), often the constable. This person could also be called a gatekeeper, although this name isn't half as good ;)