[2][3] In this context, "realm" generally means a geographic territory or political affiliation.
Contrast with "Guild versus Guild" games (or GvG, in an alternate usage from Group versus Group), such as Shadowbane or Age of Conan, in which players organize themselves into factions of their own creation and design rather than realms which are prebuilt by the game developers.
Where a single combatant or a group faces one another in PvP, RvR introduces entire factions fighting each other.
RvR was also made more interesting by bypassing the normal "red vs blue" type team battles seen in most games at the time.
[4] Mark Jacobs has more recently led a team re-imagining the Dark Age underpinnings of RvR with a new game called Camelot Unchained that is still in development.