They're two "elite" areas in GW, which can be accessed from Temple of the Ages, Zun Ku Corridor, or Chantry of Secrets. A lot of farming goes on there.
FoW - Fissure of Woe (Balthazar's dominion kinda)
UW - Underworld (Grenth's dominion)
Both Fissure of Woe and the Underworld are harder than average areas, that have many many quests to complete. It takes a bit more planning and coordination to handle these areas, and most people don't ever see them unless they are part of a guild that regularly does them.
