@ OP: Technically speaking there's only one area that counts as a dungeon and that's Dead Man's Shadow.
If your friend is talking about where a player goes to fight a boss, those are called 'Instances'
Not all areas have them. The ones that come to mind are the purple mushroom house in VG, DMP and DMS in Bills Ranch, The Doll Shrine in Zen Gardens, The Sawmill in Bass'ken, The Star Portal in Old Aquduct, Duneslam's island and Shallow Sea in Gold Beach, and the Throne Room in Otami Ruins
@ Cool: I assume you mean 'how does one obtain a large quantity of orbs during a short span of time'
Going off that assumption, the answer is farming. Depending on your level there are several areas where one can farm, both solo and in a crew. The one that comes to mind first, and is suitable for all levels is in Village Greens, by the three sand traps, on the left side of the map.