Facets
The world is split up into 5 facets.
Felucca
The facet before there were others, Felucca is the original. This facet allows non-consentual PvP and the stealing from other players. Player housing is allowed in Felucca. Resources are double here, and it is the only facet in which power scrolls spawn.
Trammel
Trammel was introduced with the Rennisance expansion. Non-consensual PvP is not allowed, nor are players stealing from players. Also, spells such as wall of stone and energy field will not impact other players in Trammel. Player housing is allowed in Trammel. Every facet except Felucca uses the basic Trammel rules set.
Ilshenar
Introduced with Third Dawn, Ilshenar was originally only able to be used with the 3D client. With the Blackthorn's Revenge expansion the 2D client was changed to allow access to Ilshenar. Player housing is not allowed here. There are several dungeons (though some players refer to the entire map as "one big dungeon"). Paragon monsters and certain artifacts spawn here.