A Traveller is someone capable of moving between the Related Worlds. Some are able to do this in physical form, which is rare, and others who do it in Spirit only while in a deep trance. Christopher merely thought that he was dreaming about travelling to the Related Worlds, but was actually travelling in his physical form. Being killed in a related world will mean that traveller will also suffer a similar death back in his world of origin. Tacroy, on the other hand, can only travel in spirit while in a deep trance, induced and stabalized by someone playing music nearby.