The world is split up into 7 facets.
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 a power scroll can be obtained at a Champion Spawn.
Trammel was introduced with the Renaissance 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, with modifications.
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. One cannot use the transportation spells Recall, Gate Travel, or Sacred Journey to get into Ilshenar. Entrance to Ilshenar is exclusively through moongate. Ilshenar's gates are located at the Shrines of Virtue and the Shrine of Chaos. One can, however, leave Ilshenar using Recall or Sacred Journey (but not Gate Travel).
Malas was introduced with the Age of Shadows expansion. Its primary features are the cities of Luna and Umbra, lots and lots of player housing, and the Dungeons Doom and Bedlam. There are several unique locations as well, such as Grimswind Ruins.
The Tokuno Islands
The Tokuno Islands are 3 islands introduced with the Samurai Empire expansion. The islands are Asian themed. Player housing is allowed, but only to a limited extent, thus making real estate there very expensive. There is one city, Zento. It has several dungeons. Use of the transporation spells Sacred Journey, Recall, and Gate Travel is unrestricted in Tokuno except in the dungeons.
Ter Mur was introduced with the Stygian Abyss expansion. It is primarily a dormant volcanic environ interspersed with lush vegetation and rivers. Inhabited natively by Gargoyles, their ruler is Queen Zhah who holds court within the walled Royal City. Other than a moongate, Ter Mur can be reached by traversing the Stygian Abyss Dungeon from Britannia.
Valley of Eodon
The Valley of Eodon was introduced with the Ultima Online: Time of Legends expansion. It draws inspiration from the classic single player game, Worlds of Ultima: Savage Empires. Through the power of the Time Lord, players are able to explore this land.