The backbone of the world is a place where all other realms leads to. At some point, the gates may have been able to provide access to other realms. But many of the gates were broken due to lack of maintenance and a number of secrets seem to be lost forever. You can find a wizard named Valo in a large town who can help you recover many of the secrets of the realm and you can also find portals for the classes and guilds of the world here.
A realm is a crafted area that can either be your private realm or an open realm where you can meet other players. Dungeons are procedurally generated areas associated with a realm that can usually only be entered by a few players per instance.